impl(oauth2): add AllowedLocations methods to minimal iam rest stub

[scotthart]

Adds methods to get the allowed locations from the IAM credentials REST endpoint.

[gemini-code-assist]

Code Review

This pull request implements the AllowedLocations functionality within the IAM Credentials REST client, supporting service accounts, workload identities, and workforce identities. It also introduces a ValidateStringArrayField utility for JSON parsing. The review feedback identifies that the AllowedLocations endpoint should use the GET method instead of POST, necessitating the removal of the request payload and updates to mock expectations in tests. Additionally, a copy-paste error was found in the unit tests for the new JSON parsing utility.

[codecov]

Codecov Report

❌ Patch coverage is 86.33333% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.68%. Comparing base (fc9f8f0) to head (24b116f).

Files with missing lines	Patch %	Lines
...ud/internal/oauth2_minimal_iam_credentials_rest.cc	56.97%	37 Missing ⚠️
...h2_impersonate_service_account_credentials_test.cc	0.00%	3 Missing ⚠️
google/cloud/internal/json_parsing.cc	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16067      +/-   ##
==========================================
- Coverage   92.69%   92.68%   -0.01%     
==========================================
  Files        2343     2343              
  Lines      216674   216959     +285     
==========================================
+ Hits       200852   201097     +245     
- Misses      15822    15862      +40     

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