impl(oauth2): subject token source for AWS credentials

[coryan]

Part of the work for #5915

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This change is 

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For commit: 3703d19b15623d7fed86cb6547eb975d72a64309

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[codecov]

Codecov Report

Base: 93.91% // Head: 93.91% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (5ccf93a) compared to base (00764bf).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #10457    +/-   ##
========================================
  Coverage   93.91%   93.91%            
========================================
  Files        1612     1612            
  Lines      146702   146808   +106     
========================================
+ Hits       137776   137879   +103     
- Misses       8926     8929     +3     

Impacted Files	Coverage Δ
...loud/internal/external_account_token_source_aws.cc	96.55% <100.00%> (+1.64%)	⬆️
...internal/external_account_token_source_aws_test.cc	99.60% <100.00%> (+0.08%)	⬆️
google/cloud/internal/async_connection_ready.cc	89.36% <0.00%> (-4.26%)	⬇️
...bigtable/examples/bigtable_hello_instance_admin.cc	81.00% <0.00%> (-2.00%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	88.29% <0.00%> (-1.01%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.82% <0.00%> (-0.84%)	⬇️
google/cloud/pubsub/samples/samples.cc	87.93% <0.00%> (+0.07%)	⬆️
...le/cloud/spanner/database_admin_connection_test.cc	99.70% <0.00%> (+0.19%)	⬆️
google/cloud/spanner/database_admin_connection.cc	85.59% <0.00%> (+0.28%)	⬆️
...torage/examples/storage_service_account_samples.cc	97.64% <0.00%> (+0.58%)	⬆️
... and 3 more

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 5ccf93a4a92dc8f28423c2aadb3d55dc6ea5e8d8

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• Kokoro Logs (requires auth)

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