Establish deprecation path for CursorResult.keys()

Establish deprecation path for CursorResult.keys()

Established a deprecation path for calling upon the
:meth:`_cursor.CursorResult.keys` method for a statement that returns no
rows to provide support for legacy patterns used by the "records" package
as well as any other non-migrated applications. Previously, this would
raise :class:`.ResourceClosedException` unconditionally in the same way as
it does when attempting to fetch rows. While this is the correct behavior
going forward, the :class:`_cursor.LegacyCursorResult` object will now in
this case return an empty list for ``.keys()`` as it did in 1.3, while also
emitting a 2.0 deprecation warning. The :class:`_cursor.CursorResult`, used
when using a 2.0-style "future" engine, will continue to raise as it does
now.

Fixes: #6427
Change-Id: I4148f28c88039e4141deeab28b1a5994e6d6e098