Add WebSocketUpgrade::selected_protocol

[coolreader18]

Motivation

This allows you to check the subprotocol before spawning the websocket task, letting you e.g. bail out with a 4XX if no subprotocol was provided.

Solution

Add a WebSocketUpgrade::selected_protocol(&self) -> Option<&HeaderValue> function, so you can access the same value as WebSocket::protocol() without yet calling on_upgrade.

[coolreader18]

Is there anyone who could take a look at this?

[jplatte]

Ping @SabrinaJewson, as the resident WebSockets and SSE expert ^^

[SabrinaJewson]

Yeah, this looks good to me. We already expose the functionality, so this is just exposing it a little earlier.