#644 fixed allowing default provider config for tenants

[Adeline09]

Description of changes: In firebase_admin/tenant_mgt.py I created the field allow_default_provider to store the default provider status of user sign-in and linked/updated its value accordingly in multiple functions. In snippets/auth/index.py I set the initial value for allow_default_provider to True on create and False on Update.

Description of tests: In integration/test_tenant_mgt.py I wrote integration tests for the initial and updated values of allow_default_provider. In tests/test_tenant_mgt.py I wrote multiple tests that toggle the status of allow_default_provider and verify its resulting value in the database.