Signal mechanism from PT processes to Tor

Once legacy/trac#28179 (moved) is done we now have bi-directional communication between the PT process and Tor itself.

We need a method where a PT process can signal to Tor different events that has occurred. This could be things like "Unable to connect to {host}:{port}".

Once we receive these messages from the PT we should expose them via the ControlPort for Tor Browser and friends to be able to act upon them.

This ticket is a tracker ticket for all of this work. Ideally we need to figure out the right way to do this mechanism (is it an update to the PT spec?) and we might also want to update goptlib to support this feature to let PT developers use it right away.