Add group assumption for OAuth authentication

[mihaimitrea-db]

Summary

Adds group assumption to the supported OAuth flows.

• External-browser OAuth sends assume_group on authorization requests.
• OAuth M2M and Databricks workload identity federation send assume_group on every token request when group_id is set.
• Browser token caches are isolated by group while existing no-group cache paths remain compatible.
• The default authentication chain can continue from an unsupported method to a supported OAuth method.

This is PR 2 of 3 and is stacked on #898. The next PR adds end-to-end WIF integration coverage.

Testing

• Full databricks-sdk-java unit test suite
• Spotless and repository push checks