extend gcp identity validation to remove identical athenz service name check

[havetisyan]

Description

Addresses issue: #3078

Contribution Checklist:

• The pull request does not introduce any breaking changes
• I have read the contribution guidelines.
• Create an issue and link to the pull request.

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Summary of Changes

Hello @havetisyan, 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 GCP identity validation process within Athenz by introducing a more flexible approach to service name matching. Previously, the Athenz service name derived from GCP project data had to precisely match the attested service name. This change now allows for discrepancies, provided that the requesting principal is explicitly authorized to assume the identity of the attested service. This modification supports broader integration patterns for GCP services within the Athenz ecosystem.

Highlights

• Flexible GCP Service Validation: The pull request relaxes the strict 1:1 mapping requirement between the requested Athenz service name (derived from GCP project and service) and the attested service name from GCP identity data. This allows for more flexible configurations.
• Authorization for Service Name Mismatches: When a mismatch occurs between the requested and attested service names, an authorization check is now performed. A principal can be authorized to 'assume identity' for a specific attested service, enabling scenarios where multiple GCP services might map to a single Athenz service.
• Authorizer Integration: The InstanceGCPProvider now includes an Authorizer component to facilitate these new identity assumption checks, using the gcp.assume_identity action.

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Code Review

This pull request extends the GCP identity validation to allow a service to assume a different Athenz service identity, provided it's authorized to do so. This is a good enhancement for flexibility. The changes in InstanceGCPProvider introduce an authorization check when the requested service name doesn't match the one in the attestation data. The accompanying tests cover the new success and failure paths.

My review includes a few suggestions:

• A critical fix to prevent a NullPointerException if the Authorizer is not configured.
• Improvements to the new test cases for better robustness and maintainability, such as using more specific exception catching and refactoring duplicated code.