implement python_impl to custom_op for basic ORM evaluator extensibility

implement python_impl to custom_op for basic ORM evaluator extensibility

Added new parameter :paramref:`_sql.Operators.op.python_impl`, available
from :meth:`_sql.Operators.op` and also when using the
:class:`_sql.Operators.custom_op` constructor directly, which allows an
in-Python evaluation function to be provided along with the custom SQL
operator. This evaluation function becomes the implementation used when the
operator object is used given plain Python objects as operands on both
sides, and in particular is compatible with the
``synchronize_session='evaluate'`` option used with
:ref:`orm_expression_update_delete`.

Fixes: #3162
Change-Id: If46ba6a0e303e2180a177ba418a8cafe9b42608e