Explicit demand control in WebSocket endpoints with only onWebSocketFrame()

[sbordet]

Jetty version(s) 12+

Description If a WebSocket endpoint only overrides onWebSocketFrame() it currently has no explicit control over the demand, because the implementation expects other methods to be present, and the other methods to control the demand explicitly.

There should be a way to explicitly control the demand if an application want to deal only with frames.

[sbordet]

See #11304.