Provide option to initiate WebSocket upgrade

Something like ServerWebSocketContainer.addEndpoint(ServerEndpointConfig) but invoked at runtime with a pre-selected ServerEndpointConfig to upgrade the current request. In other words not a permanent registration of an endpoint but rather a request to perform an upgrade with a given endpoint.

I realize this is outside JSR-356 and there is a request to update the spec WEBSOCKET_SPEC-211 but I haven't seen any activity for some time. In the mean time it would be very useful if there was some alternative in Undertow that can be used. For what it's worth Tomcat provides such an option through its WsServerContainer#doUpgrade method. WebSphere Liberty profile also provides an identical signature on its WsWsocServerContainer#doUpgrade method.

This is useful for frameworks that wish to provide unified handling of the URL space making decisions how to map requests vs having URLs predetermined at startup and registered with the JSR-356 runtime. I'm sure it would be useful to some applications as well.