title: Identity Server 4 + Angular SPA as Federation Gateway for external providers
body: <p>I'm designing the system with next components:
   - Angular SPA
   - .NET Core microservices (one of them in AuthService)</p>
  
  <p>I want to implement the architecture, where my IS4 will act as federation gateway (as client for external providers - Facebook, Microsoft, Google etc). And at the same time it must act as a provider for angular SPA (which will act a as client).</p>
  
  <p>So I want implement the next flow.
   - User clicks to social network login button (from the SPA)
   - IS4 is called and acts as a client for external provides
   - User finished the auth, external token received by IS4
   - ???
   - SPA client gets the internal tokens</p>
  
  <p>So the main stuck is how to implement the exchange token from external provider to internal one.
   - For this case how SPA should be setuped?
   - How integration between IS4 and SPA after receiving the external token should be done?
   - What parameters should be in the request from SPA in this case?
   - Could we use IS4 token endpoint and do internal token issuing by the internals of IS4?</p>