Configuration logic for onion services

We ought to have a nice clear configuration mechanism for onion services. This might want to share some of the arti configuration logic, but not necessarily all: running a proxy and an onion service in the same process is often an antifeature, unless you're doing some kind of higher-level protocol like cwtch or ricochet.

The options in C tor are not terribly well designed, but they likely cover most of what we want, and a few things we don't.