SpringBoot Allow custom RSocket WebsocketServerSpecs to be defined

See https://github.com/spring-projects/spring-framework/issues/27973 for background.

The RSocketWebSocketNettyRouteProvider auto-configuration calls HttpServerRoutes.ws to create the HttpServerRoutes. It would be nice if the WebsocketServerSpec argument could be specified easily.

Comment From: snicoll

Closing in favor of PR #29567