Create a RingCentral proxy server demo for this app

[embbnux]

Embeddable can set RingCentral server by appServer params. We can build a RingCentral proxy server, so the widget can works with RC integration in backend.

[embbnux]

Steps of Idea:

1. "https://thirdparty.com/proxy" is hosted by third party which handle RingCentral authorization and token in server side. "https://thirdparty.com/proxy" uses cookie to validate with client side, and support CORS to allow "https://ringcentral.github.io" domain.
2. set Embeddable appServer to "https://thirdparty.com/rcproxy"
3. "https://thirdparty.com/proxy" proxy all request to RingCentral server. For example, https://thirdparty.com/proxy/restapi/v1.0/account/~ to https://platform.ringcentral.com/restapi/v1.0/account/~
4. Embeddable validates authorization state by request "/proxy/restapi/v1.0/client-info". 401 for un-authorization state.
5. Embeddable request "https://thirdparty.com/proxy/logout" to revoke token.

So Embeddable can have some authorization state with "https://thirdparty.com/proxy" for SSO

[embbnux]

A lite node.js app to authorize RingCentral in server side and proxy Embeddable API request to RingCentral server https://github.com/embbnux/ringcentral-proxy
A demo to show how to use the proxy server app in Embeddable https://github.com/embbnux/ringcentral-proxy-demo
Online demo https://embbnux.github.io/ringcentral-proxy-demo/