VertexAI: Add service account key authentication support

[multiplegeorges]

What this does

This pull request adds support for authenticating to Google Vertex AI using a Service Account JSON key in addition to the existing Application Default Credentials (ADC) method. It updates both the documentation and the implementation to allow users to provide a Service Account key for Vertex AI authentication, improving flexibility and compatibility with Google Cloud best practices.

Vertex AI Authentication Enhancements:

• Added a new configuration option vertexai_service_account_key to the RubyLLM configuration, allowing users to specify a Service Account JSON key as a string for Vertex AI authentication. [1] [2] [3]
• Updated the Vertex AI provider logic to use the Service Account key for authentication if provided; otherwise, it falls back to Application Default Credentials (ADC). This includes refactoring the authorizer initialization to support both methods and defining the required OAuth scopes. [1] [2]

Documentation Updates:

• Expanded the configuration documentation to explain the new vertexai_service_account_key option, how authentication works, and Google Cloud’s recommendations regarding API keys and Service Accounts.

Test/Spec Configuration:

• Updated the test configuration to include a default value for vertexai_service_account_key, ensuring test environments can utilize the new authentication method.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Quality check

• [x ] I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes

Related issues

None.

[multiplegeorges]

Looks like #520 is making related changes to the Vertex provider, so probably a good idea to review both in tandem/together.

[crespire]

+1, could use this!

[Kcharle]

+1