Write tool for onion service health assessment

We've been getting lots of reports about bad reachability of onion services (e.g. legacy/trac#28730 (moved)) and in particular the v3 ones.

We need a tool that we can use to evaluate and monitor the health of onion services. We should use it to verify how reachable and stable onions are, and also as a benchmark for how their stability changes over time.

A relevant ticket here is legacy/trac#13209 (moved) which we can leverage in the future.

One way to write such a tool is to provide it with an onion service, and the tool fetches its desc from every HSDir, then introduces itself to all the intro points, and make sure that rendezvous can occur. Then it monitors this over time to find issues with reachability.