Client should open path for off-path path challenges from server

[flub]

We used to do this in the DISCO world too: when we received a PING we sent a PONG and also used this to trigger a new PING so the path would become open from both sides. So this is needed for feature parity.

This is easiest to describe as a specific scenario:

• Server is behind NAT/firewall.
• Client has a publicly reachable IP.
• Client connects to server over relay transport.
• Before the server has the chance to send any ADD_ADDRESS frames the client already initiates holepunching
  • Because it already has local addresses it can send in REACH_OUT frames.
  • This should speed up the direct connection
• Server sends an off-path PATH_CHALLENGE to the client's public IP, but on path 0.
• Client responds with an off-path PATH_RESPONSE.
  • But does not open a new path!

In this scenario the client should learn about a new server address from receiving the PATH_CHALLENGE. This address is already validated too. So the client should try an open that as a new path by sending it's own PATH_CHALLENGE on a new PathId, since it still needs to path-validate this path itself.

This speeds up how quickly the endpoints can switch to a direct connection.