Support Service Account Authentication

[Aurelian-Shuttleworth]

What was done

• Added --service-account <key-file> option to argparsers.py.
• Implemented auth.load_service_account using google.oauth2.service_account.
• Modified gcal.py to prioritize service account loading over user OAuth flows when the flag is present.
• Added documentation to README.md.

Why

• Headless Ops: Critical for server-side usage (CI/CD, backup scripts) where browser interaction is impossible.
• Security: Enables usage of least-privilege service accounts scoped to specific resources.

Verification Steps

Automated Tests

Ran new unit tests in the hermetic Nix environment:

$ nix develop --command pytest tests/test_service_account.py
tests/test_service_account.py ..                                       [100%]
========================= 2 passed, 1 warning in 0.02s =========================

Manual Verification (End-to-End)

Verified against the live Google Calendar API using a real Service Account:

1. Created a test Service Account via gcloud iam service-accounts create.
2. Generated a JSON key file.
3. Executed the list command:

   gcalcli --service-account key.json list

4. Result: Successfully authenticated and listed calendars (e.g., 'EU Events') without any browser interaction.

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[ClimbTheWorld]

Hi there,
Thanks for implementing this feature @Aurelian-Shuttleworth .

When is this planned to be merged?