Server : make ssh-agent available to connections

[131]

The auth-agent-req@openssh.com channel is not implemented on the server side.
(i.e, If a server wants to forward/access the auth-agent, and push some key into)

The server need to acknowledge the request by opening a channel (see openssh source code )

Also, ssh2-streams need to implement this channel ssh2-stream

I've created a PR if you could take a look