WebSocketListener anonymous classes should be invocable

[sbordet]

Jetty version
10.0.x

Description
Classes similar to:

WebSocketListener listener = new WebSocketListener() {
  ...
}

should be processed without errors by the WebSocket implementation.

They currently fail with:

java.lang.RuntimeException: Unable to access method public void org.cometd.client.websocket.jetty.JettyWebSocketTransport$JettyWebSocketDelegate.onWebSocketConnect(org.eclipse.jetty.websocket.api.Session)

probably because we are using the anonymous class rather than the interface to lookup the method handles.

See https://github.com/cometd/cometd/blob/6.0.x/cometd-java/cometd-java-server/cometd-java-server-websocket/cometd-java-server-websocket-tests/src/test/java/org/cometd/server/websocket/ConcurrentAbortPublishTest.java.