[googleapis_auth] Support impersonated_service_account in ADC

[kevmoo]

Features:

• Add support for impersonated_service_account type in Application Default Credentials (ADC).

Tests:

• Add adc_impersonation_test.dart to verify full impersonation flow.

[gemini-code-assist]

Summary of Changes

Hello @kevmoo, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the googleapis_auth package by introducing support for impersonated service accounts within Application Default Credentials (ADC), allowing for more flexible and secure authentication flows. It also improves resource management by ensuring that the quota_project_id is properly propagated for service account credentials. Additionally, a fix was applied to a configuration script to handle varying input formats, improving its reliability.

Highlights

• ADC Impersonated Service Account Support: Application Default Credentials (ADC) now supports impersonated_service_account source files, allowing clients to obtain credentials for impersonating a service account.
• Quota Project Propagation: ADC now correctly propagates the quota_project_id for Service Account credentials, ensuring that the X-Goog-User-Project header is set for requests.
• Config Script Fix: The config_delta script has been updated to correctly parse configuration files where the 'packages' key can be either a list or a map, improving its robustness.
• Refactored Client Initialization: The ImpersonatedAuthClient and clientViaServiceAccountImpersonation functions now accept an optional baseClient, providing more flexibility for transport handling.

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Changelog

• googleapis_auth/CHANGELOG.md
  • ImpersonatedAuthClient and clientViaServiceAccountImpersonation accept an optional baseClient to likely use for the transport handling.
  • Application Default Credentials (ADC) now supports impersonated_service_account source files.
  • Application Default Credentials (ADC) now propagate quota_project_id for Service Account credentials.

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[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
googleapis_auth	Non-Breaking	2.1.0	2.2.0-wip	2.2.0-wip	✔️

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Package	Status
googleapis_auth	✔️ All dependencies utilized correctly.

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[gemini-code-assist]

Code Review

This pull request adds support for impersonated_service_account in Application Default Credentials (ADC) and propagates quota_project_id for service account credentials. The changes include refactoring to handle different credential types, updating various clients and functions to support quota_project and baseClient parameters, and adding corresponding tests. The code changes are well-structured and include appropriate tests. My review includes one suggestion to improve security by using a more specific OAuth scope.

[demolaf]

Question, with these changes does the clientViaApplicationDefaultCredentials always act as one or the other? one or the other meaning source service account and target service account, if yes then that might not be the intended use of ImpersonatedAuthClient.

At least what I understand is, you should be able to create two different clients one clientViaApplicationDefaultCredentials and the other clientViaServiceAccountImpersonation and use both but now if credentials type impersonated_service_account the sign() method would always act as impersonated/target SA not source SA.

[kevmoo]

After a bunch of cleanup, all this does is wire through ADC on top of https://github.com/google/googleapis.dart/pull/711/changes

Are you saying the ADC should NOT always use the impersonation route if that's the credentials that are discovered?