title: How to integrate firebase authentication with google app engine endpoints
body: <p>I am writing a backend server for mobile applications.
  The backend is running on google app engine and written in Java.</p>
  
  <p>I want users to be able to login with federated identity such as facebook.</p>
  
  <p>I saw that google supports this kind of authentication for mobile apps via firebase authentication. What would be the best way to integrate firebase authentication with my current app engine endpoints?</p>
  
  <p>I already use the cloud platform's datastore and don't wish to work with the firebase database, only use the authentication method.</p>
  
  <p>Thanks.</p>