extend Values from HasCTE

extend Values from HasCTE

The :func:`_sql.values` construct gains a new method :meth:`_sql.Values.cte`,
which allows creation of a named, explicit-columns :class:`.CTE` against an
unnamed ``VALUES`` expression, producing a syntax that allows column-oriented
selection from a ``VALUES`` construct on modern versions of PostgreSQL, SQLite,
and MariaDB.

Fixes: #12734
Change-Id: I4a0146418420cce3cbbda4e50f5eb32206dc696b