Add pow operator support

Add pow operator support

Added support for the pow operator (``**``), with a default SQL
implementation of the ``POW()`` function.   On Oracle Database, PostgreSQL
and MSSQL it renders as ``POWER()``.   As part of this change, the operator
routes through a new first class ``func`` member :class:`_functions.pow`,
which renders on Oracle Database, PostgreSQL and MSSQL as ``POWER()``.

Fixes: #8579
Closes: #8580
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/8580
Pull-request-sha: 041b2ef474a291c6b6172e49cc6e0d548e28761a

Change-Id: I371bd44ed3e58f2d55ef705aeec7d04710c97f23