feat(auth): Add validate service account key

[bhshkh]

Description

Fixes b/411641649

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Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

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• auth/auth_validate_service_account_key.go:17, tag auth_validate_service_account_key

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[gemini-code-assist]

Summary of Changes

Hello @bhshkh, 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 enhances the authentication utilities by adding a new function, validateServiceAccountKey, designed to confirm that a provided JSON file is indeed a Google Cloud service account key. This addition improves the robustness of credential handling by explicitly checking the key's type, thereby reducing potential misconfigurations or security issues arising from using incorrect credential formats. The change is accompanied by thorough testing to ensure its reliability.

Highlights

• New Service Account Key Validation Function: Introduced validateServiceAccountKey to programmatically verify if a given JSON file is a valid Google Cloud service account key, specifically checking its type.
• Enhanced Credential Type Safety: The validation leverages google.JWTConfigFromJSON to ensure that the credential type is explicitly "service_account", preventing unintended use of other credential types like "authorized_user".
• Comprehensive Testing: A new test, TestValidateServiceAccountKey, has been added to auth_test.go to cover both successful validation of a legitimate service account key and error handling for incorrectly formatted or typed keys.

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[gemini-code-assist]

Code Review

This pull request introduces a new functionality to validate a service account key, along with corresponding tests. The implementation is straightforward and correct. I've provided a few suggestions to improve error handling, make the code example more generic, and enhance the test structure for better maintainability. Overall, this is a good addition.