feat(storage): per-operation options / Notification

[coryan]

Support per-operation google::cloud::Options for operations related to
Notification resources.

Part of the work for #7691

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: d81cfa88be611ddc9de57477a3aa9180bcf9a092

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[codecov]

Codecov Report

Merging #9205 (d81cfa8) into main (e6c39c1) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #9205      +/-   ##
==========================================
- Coverage   94.51%   94.51%   -0.01%     
==========================================
  Files        1452     1452              
  Lines      130219   130234      +15     
==========================================
+ Hits       123077   123089      +12     
- Misses       7142     7145       +3     

Impacted Files	Coverage Δ
google/cloud/storage/client.h	99.82% <100.00%> (+<0.01%)	⬆️
google/cloud/storage/client_notifications_test.cc	100.00% <100.00%> (ø)
google/cloud/pubsub/subscriber_connection_test.cc	97.22% <0.00%> (-0.70%)	⬇️
google/cloud/spanner/database_admin_connection.cc	85.14% <0.00%> (-0.58%)	⬇️
...le/cloud/spanner/database_admin_connection_test.cc	99.40% <0.00%> (-0.30%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.25% <0.00%> (+0.49%)	⬆️
...le/cloud/internal/default_completion_queue_impl.cc	97.72% <0.00%> (+0.56%)	⬆️

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[scotthart]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @coryan)