Implement ProxyJump

mosh -o ProxyJump proxy.example.com target.example.com (or similar, use ssh config?)

local mosh authenticates with proxy.example.com and starts the mosh server on proxy that forwards the packets to target.example.com.

local mosh then connects to the mosh server on proxy as usual and asks to forward a stream which then gets forwarded to target where it authenticates and finally connects to the mosh server on target.

This way, no agent forwarding is ever required since i can always connect from localhost and all agents and keys stay there.

Also lets me authenticate using my yubikey / pgp key directly with the end host, again, without ever exposing my agent og key.

todo:

• how does the proxy know the connection should end and it should exit?
• can it be multiplexed like using ssh ControlMaster to multiplex multiple connections?

The issue names no files, tests, or entry points. Start by tracing the mosh -o ProxyJump connection flow and the proxy/target authentication sequence; done requires a defined forwarding protocol, connection termination behavior, and a decision on ControlMaster-like multiplexing.