add eager_defaults="auto" for inserts

add eager_defaults="auto" for inserts

Added a new default value for the :paramref:`.Mapper.eager_defaults`
parameter "auto", which will automatically fetch table default values
during a unit of work flush, if the dialect supports RETURNING for the
INSERT being run, as well as
:ref:`insertmanyvalues <engine_insertmanyvalues>` available. Eager fetches
for server-side UPDATE defaults, which are very uncommon, continue to only
take place if :paramref:`.Mapper.eager_defaults` is set to ``True``, as
there is no batch-RETURNING form for UPDATE statements.

Fixes: #8889
Change-Id: I84b91092a37c4cd216e060513acde3eb0298abe9