[ARCHBOM-1645] Modifying anonymous_id_for_user() to handly SECRET_KEY rotation (#26162)

These changes were initially made to make it easier to do SECRET_KEY rotations.  Along the way, we found it made sense to refractor the code as well.

Changes made:
- changed get_to_create to create because now the code should only get to this block when a write is necessary
- added a lookup for anonymous_user_id. This is to return an existing anonymous_user_id rather than calculating. This will mitigate the results of SECRET_KEY rotation.
- Added monitoring to help us make better decisions: should we not sue SECRET_KEY, performance considerations...
- put old function behind toggle in case something goes wrong in production with new code
- refractoring function structure for better understanding

openedx/features/course_experience/tests/views/test_course_updates.py

@@ -49,7 +49,7 @@ class TestCourseUpdatesPage(BaseCourseUpdatesTestCase):
 
         # Fetch the view and verify that the query counts haven't changed
         # TODO: decrease query count as part of REVO-28
-        with self.assertNumQueries(52, table_blacklist=QUERY_COUNT_TABLE_BLACKLIST):
+        with self.assertNumQueries(50, table_blacklist=QUERY_COUNT_TABLE_BLACKLIST):
             with check_mongo_calls(4):
                 url = course_updates_url(self.course)
                 self.client.get(url)