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
<!-- go over following points. check them with an `x` if they do apply, (they turn into clickable checkboxes once the PR is submitted, so no need to do everything at once)

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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

1 files changed, 24 insertions, 16 deletions

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