Resolve or downgrade keymgr-related "TODO HSS" comments in arti_client and tor-hsclient

In arti_client::client, and in tor-hsclient, there are some keymgr-related TODO HSS comments. If they are blockers, we should open Onion Services: MUST tickets; otherwise, we should replace them with plain TODOs.

(Assigning to @gabi-250 as the keymgr expert.)