Multi-tenancy changes to shared Auth API requests (#539)

* Defines BaseFirebaseAuthRequestHandler class for sending Auth requests related to user management APIs and SAML/OIDC config mgmt APIs, link generation, etc.

Defines FirebaseAuthRequestHandler which extends the base class for project level only calls and which will also be extended to include tenant mgmt APIs.
Defines FirebaseTenantRequestHandler which extends the base class for tenant level only calls.

Unit tests have been modified to run tests on both subclasses.

* Addresses review comments.

* Comment clean up.

* Address more comments.

Author: bojeil-google