Add an override-endpoint-port command

[bschwind]

Someone chime in if this doesn't make sense, but while we have the option to override an endpoint (IP + port), we don't have the ability to override just the port.

This could be nice for machines that don't have a stable IP, but have port forwarding configured on their router.

[skywhale]

where do we take the IP from? do we already have a mechanism to query outside to get a global IP?

maybe override-endpoint-port to avoid confusion with the local listen port.

[bschwind]

When we run override-endpoint, we query 1.1.1.1 to get the external endpoint.

The innernet server also takes note of the IP used by the client to connect to the server.

And yeah override-endpoint-port or something similar makes sense to avoid confusion with the listening port.