Change WebSocket API to use an extractor

[davidpdrsn]

Fixes #111

Example usage:

use axum::{
    prelude::*,
    extract::ws::{WebSocketUpgrade, WebSocket},
    response::IntoResponse,
};

let app = route("/ws", get(handler));

async fn handler(ws: WebSocketUpgrade) -> impl IntoResponse {
    ws.on_upgrade(handle_socket)
}

async fn handle_socket(mut socket: WebSocket) {
    while let Some(msg) = socket.recv().await {
        let msg = if let Ok(msg) = msg {
            msg
        } else {
            // client disconnected
            return;
        };

        if socket.send(msg).await.is_err() {
            // client disconnected
            return;
        }
    }
}