generalize composite bulk insert to hybrids

generalize composite bulk insert to hybrids

Added new hybrid method :meth:`.hybrid_property.bulk_insert_setter` which
works in a similar way as :meth:`.hybrid_property.update_expression` for
bulk ORM operations.  A user-defined class method can now populate a bulk
insert mapping dictionary using the desired hybrid mechanics.   New
documentation is added showing how both of these methods can be used
including in combination with the new :func:`_sql.from_dml_column`
construct.

Fixes: #12496
Change-Id: I39f6793538f14314e0147765fa2d780b7c99493e