Jetty Websocket should have some API to handle timeouts

[lorban]

Jetty version(s) 12.0.x

Enhancement Description Currently, when WebSocketConnection.onIdleExpired or WebSocketConnection.onReadTimeout is called, the user of the API has no way to handle the timeout. CometD would be a prime user of this feature as it has its own timeout mechanism that currently clashes with the websocket one.

We should add some API with which a user could plug a timeout handler, akin to what request.addIdleTimeoutListener() works for the Jetty Core API.