Implement a mechanism for PT reachability testing

Non-vanilla bridges currently have no way to automatically test their reachability. Vanilla bridges self-test the reachability of their ORPort by creating a circuit that includes themselves, but we cannot do this for, say, obfs4. In practice, this is problematic because obfs4 operators won't know if their bridge is unreachable; for example due to NAT. In fact, BridgeDB is distributing obfs4 bridges that aren't actually reachable.

We need to build a mechanism that allows non-vanilla bridges to test their reachability. Ideally, something would create a circuit over the bridge while speaking its respective transport protocol but even a simple TCP or UDP-based reachability test would already go a long way.

Looking at the discussion over in #30331, tor seems to be the right component to trigger the reachability test. In its log files, it can then yell at the operator if the test failed. The question is: how should we design the mechanism that implements the reachability test?

One solution would be a simple HTTP API that takes as input an address, port, a transport type, and optional parameters, and then tells you if the given bridge is reachable, e.g.: the URL https://pt-reachable.torproject.org/obfs4/1.2.3.4/9002 may respond with something along the lines of obfs4_reachable: true. Ideally, if the reachability test fails, we should provide details, to help the operator figure out what went wrong.