feat(generator): generate authentication example

[coryan]

Create a minimal examples showing how to use service account
credentials for each *Client class.

Part of the work for #10114

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This change is 

[google-cloud-cpp-bot]

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Codecov Report

Base: 93.97% // Head: 93.97% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (f3be4c5) compared to base (554a496).
Patch has no changes to coverable lines.

❗ Current head f3be4c5 differs from pull request most recent head 9d944b2. Consider uploading reports for the commit 9d944b2 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #10138    +/-   ##
========================================
  Coverage   93.97%   93.97%            
========================================
  Files        1524     1523     -1     
  Lines      140332   140149   -183     
========================================
- Hits       131881   131712   -169     
+ Misses       8451     8437    -14     

Impacted Files	Coverage Δ
...ud/internal/grpc_service_account_authentication.cc	72.22% <0.00%> (-11.12%)	⬇️
google/cloud/internal/unified_grpc_credentials.cc	79.48% <0.00%> (-10.26%)	⬇️
...le/cloud/pubsub/internal/publisher_stub_factory.cc	89.47% <0.00%> (-5.27%)	⬇️
google/cloud/pubsub/subscriber_connection.cc	76.81% <0.00%> (-2.90%)	⬇️
google/cloud/pubsub/schema_admin_connection.cc	90.47% <0.00%> (-1.91%)	⬇️
...ogle/cloud/pubsub/subscription_admin_connection.cc	83.00% <0.00%> (-1.00%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	96.59% <0.00%> (-0.57%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	97.81% <0.00%> (-0.55%)	⬇️
google/cloud/storage/parallel_upload.cc	98.28% <0.00%> (-0.35%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.82% <0.00%> (-0.34%)	⬇️
... and 5 more

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Google Cloud Build Logs
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