Add Google Sheets integration for GitHub user verification

[rbren]

This PR adds support for using Google Sheets as a source for allowed GitHub usernames.

Key changes:

• Add Google Sheets client for fetching allowed usernames from a configured sheet
• Modify GitHub authentication to support both text file and Google Sheets verification
• Maintain backward compatibility with existing text file approach
• Use workload identity for Google Sheets authentication when running in GCP

Setup instructions:

1. Install required dependencies (already in pyproject.toml):

poetry install

2. Configure environment variables:

# Existing variables remain unchanged
GITHUB_CLIENT_ID=your_github_client_id
GITHUB_CLIENT_SECRET=your_github_client_secret
GITHUB_USER_LIST_FILE=/path/to/users.txt  # Optional: Keep for backwards compatibility

# New variable for Google Sheets
GITHUB_USERS_SHEET_ID=your_google_sheet_id

3. Set up Google Sheets:
   • Create a Google Sheet and note its ID from the URL
   • Put GitHub usernames in column A
   • Ensure the GCP service account has access to the sheet
   • Configure workload identity federation for your deployment environment

The system will check both text file (if configured) and Google Sheet for valid usernames.

---

To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=ghcr.io/all-hands-ai/runtime:9f26c52-nikolaik   --name openhands-app-9f26c52   ghcr.io/all-hands-ai/runtime:9f26c52

[neubig]

Looks good, but we can remove some typing imports.

[neubig]

These are not necessary, just use list and | None.