Feature request: add option to enable ssh-agent forwarding

[yshui]

Add a fn Command::forward_agent(bool) function so I can enable/disable agent forwarding when starting a remote command.

For native-mux, just need to call openssh_mux_client::SessionBuilder::agent. For process-mux IIUC we just need to pass -A to the ssh command?

[NobodyXu]

Yeah it makes sense, I'm willing to accept a PR for this feature