Websockets: Support annotated endpoints in ServerEndpointConfig

The Javadoc for javax.websocket.server.ServerEndpointConfig indicates that it supports annotation-based endpoints, however the Undertow implementation does not.

This bug report + fix allows undertow to server annotation-based websocket endpoints that are configured with programmatic configuration.

Proposed fix here: https://github.com/undertow-io/undertow/pull/694