title: firebase custom auth system : no provider id for custom token authentication, after sign in
body: <p>I'm using the firebase custom auth token system to add LinkedIn as auth provider. I have setup the Google Functions to receive the LinkedIn Access token from android over http trigger, then query LinkedIn Api to get details of user and create a user in firebase with uid <code>"linkedin:$linkedin-user-id"</code></p>
  
  <p>But now the problem is the firebase is not putting any provider id for custom auth like it do for federated providers in <code>FirebaseUser.getproviders()</code>. I need this information since my app also supports EmailAuth, GoogleAuth and FacebookAuth. Based on the provider I need to perform provider specific functions. </p>
  
  <p>Checking providers list for empty will only work if only LinkedIn is associated with user, if user has multiple accounts this check will fail.</p>
  
  <p>Is there any solution to this scenario? Firebase has probably its own reason not to include provider id for custom, but I need to some how identify the linkedin provider used for login</p>