impl(common): file-sourced subject tokens

[coryan]

To support external accounts credentials (aka BYOID) we need to support reading the subject token from a file. The file itself can be in either text or JSON format.

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: 93.83% // Head: 93.84% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (d2249f0) compared to base (2e80c50).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #10277    +/-   ##
========================================
  Coverage   93.83%   93.84%            
========================================
  Files        1573     1575     +2     
  Lines      144070   144310   +240     
========================================
+ Hits       135182   135421   +239     
- Misses       8888     8889     +1     

Impacted Files	Coverage Δ
...oud/internal/external_account_token_source_file.cc	100.00% <100.00%> (ø)
...nternal/external_account_token_source_file_test.cc	100.00% <100.00%> (ø)
google/cloud/bigtable/async_read_stream_test.cc	97.32% <0.00%> (-0.67%)	⬇️
google/cloud/pubsub/samples/samples.cc	90.70% <0.00%> (-0.08%)	⬇️
google/cloud/internal/error_metadata.h	100.00% <0.00%> (ø)
...cloud/pubsub/internal/subscription_session_test.cc	97.98% <0.00%> (+0.16%)	⬆️
google/cloud/storage/parallel_upload.cc	98.62% <0.00%> (+0.34%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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