refactor: use a CMake function to create pkg-config files

[coryan]

All generated files, and most hand-crafted files, use
a common function to create and install the pkg-config(1) files.

Part of the work for #8992

The PR is broken down in several commits to (I hope) ease the review.

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This change is 

[google-cloud-cpp-bot]

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 49aca6787b73753246ba00b5440c2c6882def668

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[codecov]

Codecov Report

Merging #9018 (49aca67) into main (83a81a0) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #9018      +/-   ##
==========================================
- Coverage   94.48%   94.47%   -0.01%     
==========================================
  Files        1422     1422              
  Lines      125254   125254              
==========================================
- Hits       118345   118336       -9     
- Misses       6909     6918       +9     

Impacted Files	Coverage Δ
generator/internal/scaffold_generator.cc	84.32% <ø> (ø)
...le/cloud/storage/internal/curl_download_request.cc	88.25% <0.00%> (-1.01%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.75% <0.00%> (-1.00%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	97.15% <0.00%> (-0.57%)	⬇️
google/cloud/pubsub/samples/samples.cc	91.97% <0.00%> (-0.08%)	⬇️

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[devbww]

The PR is broken down in several commits to (I hope) ease the review.

Thanks.