feat(pubsub): update subscription builders

[coryan]

We provide a few helpers to build the protos. While this may not be a
good idea in the long run, it is the current plan, so we need these
updates from time to time.

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #9326 (196becc) into main (da8bc9f) will increase coverage by 0.00%.
The diff coverage is 99.10%.

@@           Coverage Diff            @@
##             main    #9326    +/-   ##
========================================
  Coverage   94.66%   94.66%            
========================================
  Files        1470     1470            
  Lines      134682   134794   +112     
========================================
+ Hits       127493   127603   +110     
- Misses       7189     7191     +2     

Impacted Files	Coverage Δ
google/cloud/pubsub/snapshot_builder.h	100.00% <ø> (ø)
google/cloud/pubsub/topic_builder.h	100.00% <ø> (ø)
google/cloud/pubsub/subscription_builder.h	97.25% <97.87%> (+0.21%)	⬆️
google/cloud/pubsub/subscription_builder.cc	91.89% <100.00%> (+2.23%)	⬆️
google/cloud/pubsub/subscription_builder_test.cc	100.00% <100.00%> (ø)
...e/cloud/spanner/testing/cleanup_stale_instances.cc	72.22% <0.00%> (-5.56%)	⬇️
...integration_tests/schema_admin_integration_test.cc	98.88% <0.00%> (-1.12%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	87.87% <0.00%> (-0.68%)	⬇️
google/cloud/internal/curl_impl.cc	91.64% <0.00%> (-0.23%)	⬇️
google/cloud/pubsub/samples/samples.cc	92.18% <0.00%> (+0.07%)	⬆️
... and 4 more

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 196beccff97e2bb4a52e02f8bffb75e703d76746

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.