Fixed #35444 -- Added generic support for Aggregate.order_by.

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.

tests/aggregation/models.py

@@ -43,3 +43,7 @@ class Store(models.Model):
 
     def __str__(self):
         return self.name
+
+
+class Employee(models.Model):
+    work_day_preferences = models.JSONField()