Commit b6447211754e126f64e12fc735ad74fe557b7fb4 inadvertently
introduced a possible breaking change as it added a new argument
`iterator` and added it in between existing (potentially positional) arguments.

This moves the `iterator` argument to the end of the argument list and
requires it to be a keyword-only argument.

* feat(downloads): allow streaming downloads access to response iterator

Allow access to the underlying response iterator when downloading in
streaming mode by specifying `iterator=True`.

Update type annotations to support this change.

* docs(api-docs): add iterator example to artifact download

Document the usage of the `iterator=True` option when downloading
artifacts

* test(packages): add tests for streaming downloads

Enable the pylint check: "attribute-defined-outside-init" and fix
errors detected.

Move the `validate_attrs` function to be inside the `RequiredOptional`
class. It makes sense for it to be part of the class as it is working
on data related to the class.

add exclusive tuple to RequiredOptional data class to support for
mutually exclusive attributes

consolidate _check_missing_create_attrs and _check_missing_update_attrs
from mixins.py into _validate_attrs in utils.py

change _create_attrs in board list manager classes from
required=('label_ld',) to
exclusive=('label_id','asignee_id','milestone_id')

closes https://github.com/python-gitlab/python-gitlab/issues/1897

By having `RequiredOptional` in the `gitlab.base` module it makes it
difficult with circular imports. Move it to the `gitlab.types`
module which has no dependencies on any other gitlab module.

Create return type-hints for `RESTObject.get_id()` and
`RESTObject.encoded_id`. Previously was saying they return Any. Be
more precise in saying they can return either: None, str, or int.

utils.EncodedId() has basically the same functionalityy of using
utils._url_encode(). So remove utils._url_encode() as we don't need
it.