This moves the behaviors of `order_by` used in Postgres aggregates into
the `Aggregate` class. This allows for creating aggregate functions that
support this behavior across all database engines. This is shown by
moving the `StringAgg` class into the shared `aggregates` module and
adding support for all databases. The Postgres `StringAgg` class is now
a thin wrapper on the new shared `StringAgg` class.
Thank you Simon Charette for the review.