OnionService API questions

I think we will need to rethink the OnionService API a bit:

• we will need some way of stopping a running service (and presumably also the tasks spawned by launch). Even if we implemented stop(), restarting the stopped service wouldn't be possible, because launch() consumes unlaunched
• the onion_name() function (that implements the arti hss onion-name command) needs to be moved to OnionService. Currently, to construct an OnionService, you need to provide some objects that aren't needed when building OnionService in the "unbootrapped", unlaunchable "CLI mode" (e.g. a Runtime, a StateMgr)
• @Diziet notes that one possibility would be to rewrite OnionService using the typestate pattern

cc @nickm @Diziet