internal/v4: implement debug/status endpoint

As newly documented in the charm store.
Not all entries are implemented yet.

None of the queries in question could use existing indexes. We could add another field (a Promulgated bool) and three more indexes, but maintaining all those indexes just for this call seems like unnecessary overhead on the rest of the charm store operations. Is it faster to do one linear scan over the network or three linear scans remotely? I don't know. The other advantage the current approach brings is the possibility of finding entries that don't fit into any categories, which could be a fourth entry and would be unlikely to be indexable.