add new pattern for single inh column override

add new pattern for single inh column override

Added a new parameter :paramref:`_orm.mapped_column.use_existing_column` to
accommodate the use case of a single-table inheritance mapping that uses
the pattern of more than one subclass indicating the same column to take
place on the superclass. This pattern was previously possible by using
:func:`_orm.declared_attr` in conjunction with locating the existing column
in the ``.__table__`` of the superclass, however is now updated to work
with :func:`_orm.mapped_column` as well as with pep-484 typing, in a
simple and succinct way.

Fixes: #8822
Change-Id: I2296a4a775da976c642c86567852cdc792610eaf