Introduce WebSocket on the client and server side

[idugalic]

http://docs.spring.io/spring-framework/docs/5.0.0.BUILD-SNAPSHOT/spring-framework-reference/html/web-reactive.html#web-reactive-websocket-support

WebFlux includes reactive WebSocket client and server support. Both client and server are supported on the Java WebSocket API (JSR-356), Jetty, Undertow, Reactor Netty, and RxNetty.

On the server side, declare a WebSocketHandlerAdapter and then simply add mappings to WebSocketHandler