Add health endpoint per cache

Currently it's only possible to get the health of all user caches via the v2/cache-managers/default/caches endpoint. Instead it should be possible for a user to retrieve the health of an individual cache via v2/caches/{cacheName}?action=health}.

It's not possible to use v2/caches/{cacheName as this returns a 404 if the specified cache is not in a RUNNING state.