Fix use_existing_column with Annotated mapped_column in polymorphic inheritance

Fix use_existing_column with Annotated mapped_column in polymorphic inheritance

Fixed issue where :paramref:`_orm.mapped_column.use_existing_column`
parameter in :func:`_orm.mapped_column` would not work when the
:func:`_orm.mapped_column` is used inside of an ``Annotated`` type alias in
polymorphic inheritance scenarios. The parameter is now properly recognized
and processed during declarative mapping configuration.

Fixes: #12787
Change-Id: I0505df3f3714434e98052c4488f6b1b1d2b1f755