feat: Add GCP service account authentication support

[mporracindie]

Summary

• Add optional service account authentication as an alternative to OAuth2 refresh token flow
• Enable automated pipelines and CI/CD environments to use service accounts instead of user credentials
• Maintain full backward compatibility with existing OAuth2 configurations

Changes

New Features

• Support for service account authentication via file path (credentials_file)
• Support for service account authentication via inline JSON (credentials_json) for CI/CD environments
• Config validation to ensure mutual exclusivity between OAuth2 and service account credentials

Configuration Examples

Service account via file path:

  {                                                                                                                                                             
      "credentials_file": "/path/to/service-account-key.json",                                                                                                  
      "spreadsheet_id": "YOUR_SPREADSHEET_ID",                                                                                                                  
      "start_date": "2024-01-01T00:00:00Z",                                                                                                                     
      "user_agent": "tap-google-sheets <sa@project.iam.gserviceaccount.com>"                                                                                    
  }

Service account via inline JSON (for CI/CD):

{                                                                                                                                                             
    "credentials_json": "{\"type\": \"service_account\", ...}",                                                                                               
    "spreadsheet_id": "YOUR_SPREADSHEET_ID",                                                                                                                  
    "start_date": "2024-01-01T00:00:00Z",                                                                                                                     
    "user_agent": "tap-google-sheets <sa@project.iam.gserviceaccount.com>"                                                                                    
}

Validation Rules

• Either OAuth2 credentials OR service account credentials required (not both)
• For service accounts: either credentials_file OR credentials_json (not both)
• Existing OAuth2 configs continue working unchanged

Test Plan

• Unit tests pass (16 new tests)
• Manual testing with real service account credentials
• Discovery mode works with service account
  Sync mode extracts data correctly
• Backward compatibility verified (OAuth2 still works)

AI generated code

https://internal.qlik.dev/general/ways-of-working/code-reviews/#guidelines-for-ai-generated-code

• this PR has been written with the help of GitHub Copilot or another generative AI tool

[matt-snider]

I'm not affiliated with this project. I just wanted to integrate Google Sheets with a service account so I was very happy to find this PR. I tested this locally with a service account and works for me as documented. Thank you!