feat: support external accounts in GoogleDefaultCredentials()

[coryan]

Part of the work for #5915

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This change is 

[google-cloud-cpp-bot]

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For commit: 9a13255a70c8cfcba8c7c96dd3ca9bbe07b938b2

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[codecov]

Codecov Report

Base: 93.88% // Head: 93.88% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (5a376be) compared to base (acb4355).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10430   +/-   ##
=======================================
  Coverage   93.88%   93.88%           
=======================================
  Files        1610     1610           
  Lines      146096   146183   +87     
=======================================
+ Hits       137163   137249   +86     
- Misses       8933     8934    +1     

Impacted Files	Coverage Δ
google/cloud/internal/oauth2_google_credentials.cc	92.18% <100.00%> (+0.95%)	⬆️
...e/cloud/internal/oauth2_google_credentials_test.cc	99.38% <100.00%> (+0.19%)	⬆️
...le/cloud/internal/unified_rest_credentials_test.cc	100.00% <100.00%> (ø)
google/cloud/internal/async_connection_ready.cc	89.36% <0.00%> (-4.26%)	⬇️
...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%)	⬇️
google/cloud/storage/parallel_upload.cc	98.62% <0.00%> (+0.34%)	⬆️
...le/cloud/internal/default_completion_queue_impl.cc	97.15% <0.00%> (+0.56%)	⬆️
google/cloud/bigtable/async_read_stream_test.cc	97.99% <0.00%> (+0.66%)	⬆️
google/cloud/pubsub/subscriber_connection_test.cc	100.00% <0.00%> (+1.21%)	⬆️

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 5a376bebb0f376a5d48f77ca2c394c63e7651fcf

• Public Access Build Logs
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• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.