impl(common): a wrapper for subject tokens

[coryan]

ETOOMANYTOKENTYPES - part of the work for #5915

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This change is 

[google-cloud-cpp-bot]

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For commit: 35613f09b9e17b40dcedb4d64e9ca01fa38c4e97

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[codecov]

Codecov Report

Base: 93.81% // Head: 93.81% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (374229d) compared to base (89abc4e).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10267   +/-   ##
=======================================
  Coverage   93.81%   93.81%           
=======================================
  Files        1562     1565    +3     
  Lines      143730   143754   +24     
=======================================
+ Hits       134843   134868   +25     
+ Misses       8887     8886    -1     

Impacted Files	Coverage Δ
google/cloud/internal/subject_token.cc	100.00% <100.00%> (ø)
google/cloud/internal/subject_token.h	100.00% <100.00%> (ø)
google/cloud/internal/subject_token_test.cc	100.00% <100.00%> (ø)
...integration_tests/schema_admin_integration_test.cc	98.88% <0.00%> (-1.12%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	97.81% <0.00%> (-0.55%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.98% <0.00%> (-0.17%)	⬇️
google/cloud/pubsub/samples/samples.cc	90.70% <0.00%> (-0.08%)	⬇️
google/cloud/storage/parallel_upload.cc	98.62% <0.00%> (+0.34%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.00% <0.00%> (+2.00%)	⬆️
google/cloud/internal/async_connection_ready.cc	93.61% <0.00%> (+4.25%)	⬆️

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 374229d9d76746b1cceea5abefa0bef78806e8cd

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

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