hs: Implement self reachability test
I have not thought of the anonymity problem that could bring if any, just more of a pragmatic approach of how that could be useful.
Onion services sometimes can find themselves unable to publish a descriptor for many different reasons or even bugs that we keep finding. Even under severe DDoS, they become unavailable but they don't know.
What if they would regularly do self reachability tests and in case of failure, they could do some "recovery actions" but also dumping their state so it is more easily debuggable.
But also, they could export that status onto something like #40063 (closed) which could inform very quickly the operator which in turn could correlate with the their stats if this is a tor problem or a DDoS or network issue or anything to that end.