Add (or document) the ability to SSH through bastion hosts, if possible

This relates to #84. I'm not interested in using ssh-agent based key forwarding, but I would like to be able to jump through an intermediate host without relying on netcat.

I'm not sure how feasible this would be, but ideally you could do something like this:

    let tcp = TcpStream::connect("bastion-host:22").unwrap();
    let mut bastion_session = Session::new().unwrap();
    bastion_session.set_tcp_stream(tcp);
    bastion_session.handshake().unwrap();
    bastion_session.userauth_password("username", "password").unwrap();
    bastion_channel = bastion_session.channel_direct_tcpip("target-host", 22, None); 
    target_session = Session::new().unwrap();
    // Here's the crux of it, and where I'm really unsure of how difficult this would be
    target_session.set_tcp_stream(bastion_channel); // Or a stream from the bastion?
    target_session.handshake.unwrap();
    target_session.userauth_password("username", "password").unwrap();
    // etcetera

This follows a pattern I've used with other SSH client libraries, e.g.

    bastion_connection = await asyncssh.connect("bastion")
    target_connection = await asyncssh.connect("target", tunnel=bastion_connection)

It seems like the issue is that a channel/stream doesn't implement the RawFd trait, so we can't pass that as a TCP stream to another session. Would it be possible to implement that trait on either the channel or stream?

I'm working on my first Rust project, so please forgive my ignorance here.

EDIT: I'm also waay too used to working in a high level language, because I did a bit of digging and I doubt this will be simple. Still, it would be good to know what it might look like to do this.

Review related issue #84 and the Session, Channel, TcpStream, and RawFd APIs involved in the example. Determine whether a channel or stream can serve as the transport for a second session, then define whether the outcome should be implemented support or documentation describing the feasible approach and limitations.