refactor(iot): versioned clients

[dbolduc]

Part of the work for #10170

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This change is 

[codecov]

Codecov Report

Base: 93.65% // Head: 93.65% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (1871dca) compared to base (2735450).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10912      +/-   ##
==========================================
- Coverage   93.65%   93.65%   -0.01%     
==========================================
  Files        1717     1717              
  Lines      154743   154743              
==========================================
- Hits       144921   144919       -2     
- Misses       9822     9824       +2     

Impacted Files	Coverage Δ
google/cloud/internal/timer_queue.cc	96.10% <0.00%> (-1.30%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	98.78% <0.00%> (-1.22%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	84.21% <0.00%> (-0.66%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	96.59% <0.00%> (-0.57%)	⬇️
google/cloud/storage/parallel_upload.cc	98.28% <0.00%> (-0.35%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.83% <0.00%> (ø)
google/cloud/pubsub/samples/samples.cc	87.01% <0.00%> (+0.07%)	⬆️
...le/cloud/storage/internal/curl_download_request.cc	89.29% <0.00%> (+0.66%)	⬆️

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