Usage:
- enter username and press connect in multiple tabs
- enter receiver username and press send.
Resources: https://docs.spring.io/spring-framework/reference/web/websocket/stomp/message-flow.html https://www.baeldung.com/spring-websockets-send-message-to-user https://huongdanjava.com/send-stomp-message-to-a-specific-user-with-spring-websocket.html https://docs.spring.io/spring-framework/reference/web/websocket/stomp/user-destination.html