Fix scopes in OAuthClient with using model serving on Azure

[sdonohoo-db]

Changes

If the application instantiating an instance of OAuthClient is using Azure, full access to Azure Databricks is required. So the scope list must include '{effective_azure_login_app_id}/user_impersonation' and 'offline_access'. However, the scopes were set to these instead of appending to any existing scopes that may be been in the list. In the case of calling a model serving endpoint, scope 'all-apis' is also needed.

Changed the line to append the additional Azure-required scopes if not already present in the list rather than set.

Tests

Tested using https://github.com/sdonohoo-db/model-serving-oauth/blob/main/U2M/flask_app_with_oauth.py sample client adapted from https://github.com/databricks/databricks-sdk-py/blob/main/examples/flask_app_with_oauth.py to demonstrate using U2M OAuth with model serving.

• make test run locally
• make fmt applied
• relevant integration tests applied