Bring websocket handling more inline with handler API

[davidpdrsn]

Feature Request

Motivation

Currently handling websockets works quite differently from handling other requests. This has a few downsides:

• You currently cannot return an error from a ws request. The connection will always be upgraded. This was also highlighted in WebSocketHandler::call should be allowed to fail #73.
• You cannot customize the connection upgrade response. Users might wanna add additional headers. That is currently not possible.
• Applying extractors to ws requests is possible but the implementation isn't very clear. It requires code very similar to what is already in handler/mod.rs.

Proposal

Change the websocket API to instead use a regular handler and a special IntoResponse. Something like

// build our application with a route
let app = route("/", get(handler));

async fn ws_handler(
    // this extractor checks for the required headers
    ws: extract::WebSocketUpgrade,
    // you can put other extractors here
) -> impl IntoResponse {
    // here you can do whatever you want to prepare for handling the connection
    // possibly return an error and rejection the connection all together

    // actually upgrade the connection and call the async closure with
    // the websocket connection when we have it
    ws.on_upgrade(|socket: WebSocket| async {
        // do stuff...
    })
}

This is similar to how things work in actix.

This solution was suggest by gbaranski#5119 on Discord.

Alternatives

Leads to slightly more code in the basic case where you don't wanna apply any extractors and just wanna start a ws connection without rejections. I think that tradeoff is worth it though.