Add a status API to OnionService.
(This doesn't yet send any status events.)
Begins work on #1083 (closed).
Assigning review to @gabi-250 since I think you're the next one to get this; do you think it's a reasonable foundation? If so I'd suggest we merge it and pass #1083 (closed) to you and/or @Diziet, if you're so inclined.