Add DatabricksOAuthTokenSource

[emmyzhou-db]

What changes are proposed in this pull request?

• Implemented DatabricksOAuthTokenSource class that handles the OAuth token exchange flow
• This class manages the OAuth token exchange flow using ID tokens to obtain access tokens
• The token exchange mechanism is essential for implementing OIDC-based authentication in Databricks

How is this tested?

The implementation is tested through unit tests that:

• Mock the ID Token Source to simulate the token exchange flow
• Mock the HTTP client to verify token exchange requests and responses
• Test the complete token exchange flow including audience selection and error handling
• Validate proper parameter handling and error responses
• All tests are automated and part of the unit test suite. No manual testing was required.

NO_CHANGELOG=true

[github-actions[bot]]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger: go/deco-tests-run/sdk-java

Inputs:

• PR number: 439
• Commit SHA: 28dca4f31b963e76cc4a7f3c7cb35573239bb79e

Checks will be approved automatically on success.