Jetty 12 - Add a WebSocket MessageSink for Flow.Publisher

[sbordet]

Jetty version(s)
12+

Enhancement Description
As we have MessageSink for InputStream, we could have an implementation for Flow.Publisher<ByteBuffer> since we cannot directly have Content.Source due to classloading issues.

@WebSocket
public class WSEndpoint {
  @OnWebSocketMessage
  public void onMessage(Flow.Publisher<ByteBuffer> publisher) {
    // Consume the message asynchronously.
  }
}

[github-actions]

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