Implement and test a WebSocket Proxy with the 9.4 Jetty API

[lachlan-roberts]

Adding a simple WebSocket to tests using the WebSocketPartialListener and WebSocketPingPongListener interfaces.

• Text and binary frames are simply forwarded through to the other remote endpoint.
• Ping and Pong frames are more complex as the implementation forces us to send the pong response when the onPing() method exits.

[sbordet]

LGTM. Just write an additional test for the idle timeout case: connect, echo a message, then wait until the idle timeout expires => you should see a the connections being closed.