impl(oauth2): parsing for AWS subject token source

[coryan]

AWS has a fairly complicated protocol to obtain subject token. This PR just introduces parsing from the JSON object configuration to a C++ structure. Future PRs will use this structure to drive the HTTP flows required to get these credentials.

Part of the work for #5915

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Base: 93.89% // Head: 93.89% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (3cac3ca) compared to base (80e686d).
Patch coverage: 98.63% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10432      +/-   ##
==========================================
- Coverage   93.89%   93.89%   -0.01%     
==========================================
  Files        1610     1612       +2     
  Lines      146183   146330     +147     
==========================================
+ Hits       137257   137391     +134     
- Misses       8926     8939      +13     

Impacted Files	Coverage Δ
...loud/internal/external_account_token_source_aws.cc	93.93% <93.93%> (ø)
...internal/external_account_token_source_aws_test.cc	100.00% <100.00%> (ø)
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	97.05% <0.00%> (-2.95%)	⬇️
...torage/examples/storage_service_account_samples.cc	96.47% <0.00%> (-1.18%)	⬇️
...integration_tests/schema_admin_integration_test.cc	98.88% <0.00%> (-1.12%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.99% <0.00%> (-0.67%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	84.21% <0.00%> (-0.66%)	⬇️
google/cloud/storage/parallel_upload.cc	98.28% <0.00%> (-0.35%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	89.29% <0.00%> (-0.34%)	⬇️

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