res_sorcery_memory_cache: Add support for a full backend cache.

This change introduces the configuration option 'full_backend_cache'
which changes the cache to be a full mirror of the backend instead
of a per-object cache. This allows all sorcery retrieval operations
to be carried out against it and is useful for object types which
are used in a "retrieve all" or "retrieve some" pattern.

ASTERISK-25625 #close

Change-Id: Ie2993487e9c19de563413ad5561c7403b48caab5