Allow custom sync session class in ``AsyncSession``.

Allow custom sync session class in ``AsyncSession``.

The :class:`_asyncio.AsyncSession` now supports overriding which
:class:`_orm.Session` it uses as the proxied instance. A custom ``Session``
class can be passed using the :paramref:`.AsyncSession.sync_session_class`
parameter or by subclassing the ``AsyncSession`` and specifying a custom
:attr:`.AsyncSession.sync_session_class`.

Fixes: #6689
Change-Id: Idf9c24eae6c9f4e2fff292ed748feaa449a8deaa