feat(common): factory function for external accounts

[coryan]

Fixes #5915

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 527b20348176f976fcad2c1802f5c8abb2206219

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

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

[codecov]

Codecov Report

Base: 93.87% // Head: 93.88% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (527b203) compared to base (b191e83).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10465   +/-   ##
=======================================
  Coverage   93.87%   93.88%           
=======================================
  Files        1624     1624           
  Lines      147462   147465    +3     
=======================================
+ Hits       138436   138448   +12     
+ Misses       9026     9017    -9     

Impacted Files	Coverage Δ
google/cloud/internal/credentials_impl_test.cc	100.00% <ø> (ø)
google/cloud/credentials.cc	100.00% <100.00%> (ø)
google/cloud/internal/async_connection_ready.cc	89.36% <0.00%> (-4.26%)	⬇️
google/cloud/completion_queue_test.cc	97.13% <0.00%> (-0.20%)	⬇️
google/cloud/pubsub/samples/samples.cc	88.00% <0.00%> (+0.14%)	⬆️
...sub/internal/batching_publisher_connection_test.cc	99.62% <0.00%> (+0.18%)	⬆️
...le/cloud/storage/internal/curl_download_request.cc	89.63% <0.00%> (+0.33%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	99.33% <0.00%> (+1.33%)	⬆️

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