feat(auth): Simplify and unify Workload Identity configuration

[jerop]

Renames to to create a single, unified variable for all Google Cloud authentication.

Key changes:

• The now uses this unified provider for a more generic authentication step.
• All documentation and workflows have been updated to use the new variable.
• The script is updated to reflect the new variable name in its output.

This is a follow up after #18

[gemini-cli]

📋 Review Summary

This pull request introduces a significant improvement by renaming OTLP_GCP_WIF_PROVIDER to GCP_WIF_PROVIDER, creating a single, unified variable for all Google Cloud authentication. This change simplifies the configuration for users and enhances the maintainability of the action. The updates are consistently applied across all relevant documentation, workflows, and scripts.

🔍 General Feedback

The changes are clear, concise, and well-executed. The renaming of the variable to be more generic is a good architectural decision that will make the action easier to understand and configure. The accompanying updates to documentation and helper scripts are thorough and ensure a smooth transition for users.

🎯 Specific Feedback

No specific issues identified in this review.

✅ Highlights

• Simplification: Unifying the WIF provider variable under GCP_WIF_PROVIDER is an excellent simplification. It reduces redundancy and makes the configuration more intuitive.
• Consistency: The change was applied consistently across all files, including workflow definitions, documentation, and the setup script. This attention to detail is commendable.
• Clarity in action.yml: Renaming the authentication step from "Authenticate to Google Cloud for Telemetry" to the more general "Authenticate to Google Cloud" accurately reflects its new, broader purpose.
• Improved Script Output: The setup_workload_identity.sh script now provides clearer output, explicitly listing the aiplatform.user permission and using the new variable names, which will greatly help users during setup.