Comments on (named) constraints

Adds support for comments on named constraints, including `ForeignKeyConstraint`, `PrimaryKeyConstraint`, `CheckConstraint`, `UniqueConstraint`, solving the [Issue 5667](https://github.com/sqlalchemy/sqlalchemy/issues/5667).

Supports only PostgreSQL backend.

### Description

Following the example of [Issue 1546](https://github.com/sqlalchemy/sqlalchemy/issues/1546), supports comments on constraints. Specifically, enables comments on _named_ ones — as I get it, PostgreSQL prohibits comments on unnamed constraints.

Enables setting the comments for named constraints like this:
```
Table(
   'example', metadata,
   Column('id', Integer),
   Column('data', sa.String(30)),
   PrimaryKeyConstraint(
       "id", name="id_pk", comment="id_pk comment"
    ),
   CheckConstraint('id < 100', name="cc1", comment="Id value can't exceed 100"),
   UniqueConstraint(['data'], name="uc1", comment="Must have unique data field"),
)
```

Provides the DDL representation for constraint comments and routines to create and drop them. Class `.Inspector` reflects constraint comments via methods like `get_check_constraints` .
### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
- [ ] A short code fix
- [x] A new feature implementation
	- Solves the issue 5667.
	- The commit message includes `Fixes: 5667`.
	- Includes tests based on comment reflection.

**Have a nice day!**

Fixes: #5667
Closes: #7742
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7742
Pull-request-sha: 42a5d3c3e9ccf9a9d5397fd007aeab0854f66130

Change-Id: Ia60f578595afdbd6089541c9a00e37997ef78ad3

3 files changed, 44 insertions, 18 deletions

diff --git a/lib/sqlalchemy/engine/default.py b/lib/sqlalchemy/engine/default.py
index 40af06252..5d3ff8bb7 100644
--- a/lib/sqlalchemy/engine/default.py
+++ b/lib/sqlalchemy/engine/default.py
@@ -113,6 +113,7 @@ class DefaultDialect(Dialect):
     preparer = compiler.IdentifierPreparer
     supports_alter = True
     supports_comments = False
+    supports_constraint_comments = False
     inline_comments = False
     supports_statement_cache = True
 
diff --git a/lib/sqlalchemy/engine/interfaces.py b/lib/sqlalchemy/engine/interfaces.py
index 28ed03f99..c33666e4b 100644
--- a/lib/sqlalchemy/engine/interfaces.py
+++ b/lib/sqlalchemy/engine/interfaces.py
@@ -357,7 +357,21 @@ class ReflectedColumn(TypedDict):
     object"""
 
 
-class ReflectedCheckConstraint(TypedDict):
+class ReflectedConstraint(TypedDict):
+    """Dictionary representing the reflected elements corresponding to
+    :class:`.Constraint`
+
+    A base class for all constraints
+    """
+
+    name: Optional[str]
+    """constraint name"""
+
+    comment: NotRequired[Optional[str]]
+    """comment for the constraint, if present"""
+
+
+class ReflectedCheckConstraint(ReflectedConstraint):
     """Dictionary representing the reflected elements corresponding to
     :class:`.CheckConstraint`.
 
@@ -366,9 +380,6 @@ class ReflectedCheckConstraint(TypedDict):
 
     """
 
-    name: Optional[str]
-    """constraint name"""
-
     sqltext: str
     """the check constraint's SQL expression"""
 
@@ -377,7 +388,7 @@ class ReflectedCheckConstraint(TypedDict):
     object"""
 
 
-class ReflectedUniqueConstraint(TypedDict):
+class ReflectedUniqueConstraint(ReflectedConstraint):
     """Dictionary representing the reflected elements corresponding to
     :class:`.UniqueConstraint`.
 
@@ -386,9 +397,6 @@ class ReflectedUniqueConstraint(TypedDict):
 
     """
 
-    name: Optional[str]
-    """constraint name"""
-
     column_names: List[str]
     """column names which comprise the constraint"""
 
@@ -400,7 +408,7 @@ class ReflectedUniqueConstraint(TypedDict):
     object"""
 
 
-class ReflectedPrimaryKeyConstraint(TypedDict):
+class ReflectedPrimaryKeyConstraint(ReflectedConstraint):
     """Dictionary representing the reflected elements corresponding to
     :class:`.PrimaryKeyConstraint`.
 
@@ -409,9 +417,6 @@ class ReflectedPrimaryKeyConstraint(TypedDict):
 
     """
 
-    name: Optional[str]
-    """constraint name"""
-
     constrained_columns: List[str]
     """column names which comprise the constraint"""
 
@@ -420,7 +425,7 @@ class ReflectedPrimaryKeyConstraint(TypedDict):
     object"""
 
 
-class ReflectedForeignKeyConstraint(TypedDict):
+class ReflectedForeignKeyConstraint(ReflectedConstraint):
     """Dictionary representing the reflected elements corresponding to
     :class:`.ForeignKeyConstraint`.
 
@@ -429,9 +434,6 @@ class ReflectedForeignKeyConstraint(TypedDict):
 
     """
 
-    name: Optional[str]
-    """constraint name"""
-
     constrained_columns: List[str]
     """local column names which comprise the constraint"""
 
@@ -888,6 +890,12 @@ class Dialect(EventTarget):
     definition of a Table or Column.  If False, this implies that ALTER must
     be used to set table and column comments."""
 
+    supports_constraint_comments: bool
+    """Indicates if the dialect supports comment DDL on constraints.
+
+    .. versionadded: 2.0
+    """
+
     _has_events = False
 
     supports_statement_cache: bool = True
diff --git a/lib/sqlalchemy/engine/reflection.py b/lib/sqlalchemy/engine/reflection.py
index 655a9f5c1..9bac97db0 100644
--- a/lib/sqlalchemy/engine/reflection.py
+++ b/lib/sqlalchemy/engine/reflection.py
@@ -974,6 +974,9 @@ class Inspector(inspection.Inspectable["Inspector"]):
         * ``name`` -
           optional name of the primary key constraint.
 
+        * ``comment`` -
+          optional comment on the primary key constraint.
+
         :param table_name: string name of the table.  For special quoting,
          use :class:`.quoted_name`.
 
@@ -1073,6 +1076,9 @@ class Inspector(inspection.Inspectable["Inspector"]):
         * ``name`` -
           optional name of the foreign key constraint.
 
+        * ``comment`` -
+          optional comment on the foreign key constraint
+
         :param table_name: string name of the table.  For special quoting,
          use :class:`.quoted_name`.
 
@@ -1273,6 +1279,9 @@ class Inspector(inspection.Inspectable["Inspector"]):
         * ``column_names`` -
           list of column names in order
 
+        * ``comment`` -
+          optional comment on the constraint
+
         :param table_name: string name of the table.  For special quoting,
          use :class:`.quoted_name`.
 
@@ -1462,6 +1471,9 @@ class Inspector(inspection.Inspectable["Inspector"]):
           may or may not be present; a dictionary with additional
           dialect-specific options for this CHECK constraint
 
+        * ``comment`` -
+          optional comment on the constraint
+
           .. versionadded:: 1.3.8
 
         :param table_name: string name of the table.  For special quoting,
@@ -1785,8 +1797,9 @@ class Inspector(inspection.Inspectable["Inspector"]):
                 if pk in cols_by_orig_name and pk not in exclude_columns
             ]
 
-            # update pk constraint name
+            # update pk constraint name and comment
             table.primary_key.name = pk_cons.get("name")
+            table.primary_key.comment = pk_cons.get("comment", None)
 
             # tell the PKConstraint to re-initialize
             # its column collection
@@ -1867,6 +1880,7 @@ class Inspector(inspection.Inspectable["Inspector"]):
                     refspec,
                     conname,
                     link_to_name=True,
+                    comment=fkey_d.get("comment"),
                     **options,
                 )
             )
@@ -1948,6 +1962,7 @@ class Inspector(inspection.Inspectable["Inspector"]):
         for const_d in constraints:
             conname = const_d["name"]
             columns = const_d["column_names"]
+            comment = const_d.get("comment")
             duplicates = const_d.get("duplicates_index")
             if include_columns and not set(columns).issubset(include_columns):
                 continue
@@ -1971,7 +1986,9 @@ class Inspector(inspection.Inspectable["Inspector"]):
                 else:
                     constrained_cols.append(constrained_col)
             table.append_constraint(
-                sa_schema.UniqueConstraint(*constrained_cols, name=conname)
+                sa_schema.UniqueConstraint(
+                    *constrained_cols, name=conname, comment=comment
+                )
             )
 
     def _reflect_check_constraints(