CORS settings option for socket.io

### Description

With current node-red-dashboard , there is not a CORS setting for the ui socket.

I'm currently writing Web Components (Lit Element) clones of the dashboard ui elements with the plan of loading then from a different domain and connecting via a WebRTC bridge wrtten in node-js. When i tried to implement a browser aware version of the bridge, I ran into the CORS problem. A settings.js entry to pass an aditional parameter to socketio(server,....) in ui.js would be great!

### Have you provided an initial effort estimate for this issue?