Support VIRTUAL computed columns on PostgreSQL.

Support VIRTUAL computed columns on PostgreSQL.

Support for ``VIRTUAL`` computed columns on PostgreSQL 18 and later has
been added. The default behavior when :paramref:`.Computed.persisted` is
not specified has been changed to align with PostgreSQL 18's default of
``VIRTUAL``. When :paramref:`.Computed.persisted` is not specified, no
keyword is rendered on PostgreSQL 18 and later; on older versions a
warning is emitted and ``STORED`` is used as the default. To explicitly
request ``STORED`` behavior on all PostgreSQL versions, specify
``persisted=True``.

Fixes: #12866
Change-Id: Ic2ebdbe79e230a88370cf2b3503d2d1815f72a39