feat(generator): generate simple samples for *Client

[coryan]

This is part of the work for #10114. Future PRs will include these samples in the CI builds, update the scaffold to generate build scripts for them, and inject the samples into the Doxygen landing page. I don't want to create PRs that change a thousand files at a time.

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Codecov Report

Base: 93.97% // Head: 93.98% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (a485033) compared to base (c50cb0c).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head a485033 differs from pull request most recent head 5b79693. Consider uploading reports for the commit 5b79693 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10118   +/-   ##
=======================================
  Coverage   93.97%   93.98%           
=======================================
  Files        1515     1517    +2     
  Lines      140003   140021   +18     
=======================================
+ Hits       131566   131593   +27     
+ Misses       8437     8428    -9     

Impacted Files	Coverage Δ
generator/generator_test.cc	94.36% <100.00%> (ø)
generator/internal/descriptor_utils.cc	88.46% <100.00%> (+0.08%)	⬆️
generator/internal/sample_generator.cc	100.00% <100.00%> (ø)
generator/internal/sample_generator.h	100.00% <100.00%> (ø)
google/cloud/internal/async_connection_ready.cc	89.36% <0.00%> (-4.26%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.65% <0.00%> (+0.16%)	⬆️
...le/cloud/storage/internal/curl_download_request.cc	89.63% <0.00%> (+0.33%)	⬆️
google/cloud/pubsub/subscriber_connection_test.cc	98.36% <0.00%> (+0.54%)	⬆️
...le/cloud/internal/default_completion_queue_impl.cc	97.15% <0.00%> (+0.56%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.00% <0.00%> (+2.00%)	⬆️
... and 1 more

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