arti-client level APIs for running onion services

We should have APIs at the level of arti-client for launching onion services. These should either work off of a persistent configuration and persistent state, or off of objects that the caller gets to manage itself (as in "ephemeral onions" in the C tor implementation).

It seems that, if we have the configured onion services handled via our usual configuration mechanism, the remaining task would be to have something allowing the user to create an onion service from the appropriate configuration object, returning a handle to that onion service using the correct API. This could implement both persistent and ephemeral onion services.