title: Firebase admin - get Google OAuth token
body: <p>I have a web application where users can <code>sign in with Google</code>.</p>
  <p>To the sign-in process, I add a scope to be able to access Google Calendar.</p>
  <p>Now that the user is signed in, I would like to - in server-side - get their current Google access token in order to make a request and get a list of their events.</p>
  <p>Is there a way to get the current OAuth token (no need for refresh token) in order for me to make this completely on the server-side?</p>