API and CLI for obtaining K_hsid

Probably this should be logged at level DEBUG at least. Without this, we'll not really be able to do an ad-hoc test since we won't know what .onion to try to connect to.

We log a newly-generated K_hsid since !1689 (merged), but there should be:

1. An API that lets you get the K_hsid from a running hidden service (if it knows it, which it might not...)
2. A CLI operation that does the above