column_descriptions or equiv for DML, core select

column_descriptions or equiv for DML, core select

Added new attributes :attr:`.ValuesBase.returning_column_descriptions` and
:attr:`.ValuesBase.entity_description` to allow for inspection of ORM
attributes and entities that are installed as part of an :class:`.Insert`,
:class:`.Update`, or :class:`.Delete` construct. The
:attr:`.Select.column_descriptions` accessor is also now implemented for
Core-only selectables.

Fixes: #7861
Change-Id: Ia6a1cd24c798ba61f4e8e8eac90a0fd00d738342