Jetty 12 - Add a WebSocket MessageSink for Flow.Publisher

**Jetty version(s)**
12+

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

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