Create server util to allow pulling requests

The current server API is callback-based, where Service is called whenever a request is received. Some applications need to invert it such that they take a request only when they are ready. This proposes creating a utility in hyper-util that would allow usage similar to this:

// naming is hard
let (svc, rx) = hyper_util::server::puller();

let conn = Http::new().serve_connection(socket, svc);
tokio::spawn(conn);

// pull off requests
while let Some((req, tx)) = rx.next().await? {
    println!("{:?}", req);
    tx.respond(Response::new(Empty::new());
}

Start by reading the server Service API and the hyper-util entry points around Http::new().serve_connection. Define the puller’s request and response-channel behavior, including readiness and shutdown, then verify that it supports the usage shown with tokio::spawn and an async request loop.