refactor(iam): versioned v1 clients

[dbolduc]

Part of the work for #10170

This one was a mixed library, that also has a handwritten backwards compatibility retry_traits.h.

I am not going to update the samples to use versioned clients yet. I think cloud docs use region tags at HEAD, so it would be wise for us to wait for a release with the versioned clients before updating the samples.

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This change is 

[codecov]

Codecov Report

Base: 93.64% // Head: 93.64% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (ca4ef1a) compared to base (2ec044f).
Patch has no changes to coverable lines.

❗ Current head ca4ef1a differs from pull request most recent head dd28a3a. Consider uploading reports for the commit dd28a3a to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10944   +/-   ##
=======================================
  Coverage   93.64%   93.64%           
=======================================
  Files        1717     1717           
  Lines      154878   154890   +12     
=======================================
+ Hits       145037   145054   +17     
+ Misses       9841     9836    -5     

Impacted Files	Coverage Δ
google/cloud/pubsub/samples/iam_samples.cc	94.70% <ø> (ø)
...le/cloud/storage/internal/curl_download_request.cc	88.62% <0.00%> (-0.34%)	⬇️
google/cloud/spanner/read_partition.h	100.00% <0.00%> (ø)
google/cloud/spanner/samples/samples.cc	62.27% <0.00%> (ø)
google/cloud/spanner/internal/session_pool.cc	88.11% <0.00%> (ø)
google/cloud/spanner/client.cc	98.15% <0.00%> (+0.10%)	⬆️
google/cloud/spanner/read_partition.cc	85.00% <0.00%> (+0.38%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.49% <0.00%> (+0.50%)	⬆️
...ud/spanner/integration_tests/client_stress_test.cc	84.86% <0.00%> (+0.65%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.00% <0.00%> (+2.00%)	⬆️

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No region tags are edited in this PR.

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[dbolduc]

Oops, I messed up some of the handwritten samples. This PR needs some more work before review.