[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