Support sending custom global requests from the client

[ChrisJr404]

This covers the client side of #417. Right now there's no way to send a global request with a non-standard name, so the config request from that issue (and anything else outside the built-in forwarding/keepalive ones) isn't reachable.

I added Handle::send_global_request(name, data, want_reply) that follows the same pattern as tcpip_forward and friends: it pushes the request onto the wire and, when want_reply is set, correlates the SUCCESS/FAILURE reply back through a oneshot. On success you get the response-specific payload back (empty if the peer sent none), and a failure comes back as RequestDenied. With want_reply false it just sends and returns Ok(None).

let reply = handle.send_global_request("config", payload, true).await?;

There's a round-trip test in client/test.rs against the test server, which replies with a failure for the unknown name.

I kept this to the client send path since that's what the issue asks for. Handling incoming custom requests could be a nice follow-up.