support add_cte() for TextualSelect

support add_cte() for TextualSelect

Fixed issue where the :meth:`.HasCTE.add_cte` method as called upon a
:class:`.TextualSelect` instance was not being accommodated by the SQL
compiler. The fix additionally adds more "SELECT"-like compiler behavior to
:class:`.TextualSelect` including that DML CTEs such as UPDATE and INSERT
may be accommodated.

Fixes: #7760
Change-Id: Id97062d882e9b2a81b8e31c2bfaa9cfc5f77d5c1
(cherry picked from commit bef67e58121704a9836e1e5ec2d361cd2086036c)