Extend tor-fw-helper spec to support arbitrary ports.
Currently, tor-fw-helper accepts two ports, the DirPort and the ORPort, and forwards them, reporting back success or failure.
In #4567 (moved) we had to extend tor-fw-helper to forward the ports of pluggable transport proxies. To do that, we decided to change the interface of tor-fw-helper. We now provide it with a list of ports to-be-forwarded, and the tor-fw-helper reports back which operations succeeded and which failed.
This ticket is responsible for providing a specification for this new interface.