HandlerMapping for WebSocket Requests Only

[rstoyanchev]

There is an existing WebSocketUrlHandlerMapping in spring-websocket (none in spring-webflux). However currently it does nothing more than delegate Llifecycle methods to handlers. What would be useful is the ability to map WebSocket requests only. That avoids the need for a dedicated path or sub-path specifically for WebSocket requests and makes it possible for the same URL can be handled differently when it isn't a WebSocket request.