Provide service account creds to gcloud from environment variables

[schnecle]

Allow customers to pass Google Service Account credentials directly via an environment variable following https://github.com/googleapis/google-auth-library-nodejs?tab=readme-ov-file#loading-credentials-from-environment-variables

Checklist (if applicable):

• [x] Tested (manually, unit tested, etc.) Updated the cat-eval example to validate Firestore, GCloud, and Vertex
• [ ] Changelog updated
• [x] Docs updated Added docs around the new option to pass in credentials directly via environment variable