Add FunctionElement.aggregate_order_by

Add FunctionElement.aggregate_order_by

Added new generalized aggregate function ordering to functions via the
:func:`_functions.FunctionElement.aggregate_order_by` method, which
receives an expression and generates the appropriate embedded "ORDER BY" or
"WITHIN GROUP (ORDER BY)" phrase depending on backend database.  This new
function supersedes the use of the PostgreSQL
:func:`_postgresql.aggregate_order_by` function, which remains present for
backward compatibility.   To complement the new parameter, the
:paramref:`_functions.aggregate_strings.order_by` which adds ORDER BY
capability to the :class:`_functions.aggregate_strings` dialect-agnostic
function which works for all included backends. Thanks much to Reuven
Starodubski with help on this patch.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #12853
Closes: #12856
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12856
Pull-request-sha: d93fb591751227eb1f96052ea3ad449f511f70b3

Change-Id: I8eb41ff2d57695963a358b5f0017ca9372f15f70