Add a way to manually upgrade a request to WebSocket

[wacumov]

Adds a way to manually upgrade an incoming request to WebSocket (#2235).

app.get("socket") { req in
    // Check things from the request.
    return req.webSocket { req, ws in
        // Handle WebSocket lifecycle here
    }
}

[flowtoolz]

@tanner0101 I'd be super excited to see this happen, maybe you know someone who can review it ... 😁

[0xTim]

@wacumov thanks for this and sorry it's languished for so long! If you could fix the conflicts and add a test to show the manual upgrade we can get this merged!

[wacumov]

@0xTim I fixed the conflicts and added a test for the manual upgrade.

[0xTim]

Thanks!

[VaporBot]

These changes are now available in 4.42.0