Support accepting remote connections through SSH tunnel

[clue]

Currently, this project has a focus on creating outgoing connection which can be used by higher-level protocol implementations.

In the future, we should also support accepting incoming connections through an SSH tunnel. This can be implemented by starting a local socket server and using ssh -R 0.0.0.0:8080:localhost:1234 [email protected] and possibly by parsing its debugging output to get access to actual remote client IPs. I've started looking into this and will keep this ticket updated as I make progress.