doc(common): add Doxygen group for common options

[coryan]

Fixes #10130

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This change is 

[google-cloud-cpp-bot]

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[google-cloud-cpp-bot]

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For commit: 305722f9b56d819cf0b69133e3d403a203a11d95

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[codecov]

Codecov Report

Base: 93.97% // Head: 93.96% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (400ec14) compared to base (71b11b6).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10192      +/-   ##
==========================================
- Coverage   93.97%   93.96%   -0.01%     
==========================================
  Files        1526     1526              
  Lines      140600   140600              
==========================================
- Hits       132130   132120      -10     
- Misses       8470     8480      +10     

Impacted Files	Coverage Δ
google/cloud/options.h	97.82% <ø> (ø)
...e/cloud/spanner/testing/cleanup_stale_instances.cc	68.85% <0.00%> (-6.56%)	⬇️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	97.05% <0.00%> (-2.95%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	97.82% <0.00%> (-0.84%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	96.59% <0.00%> (-0.57%)	⬇️
google/cloud/completion_queue_test.cc	97.13% <0.00%> (-0.20%)	⬇️
...sub/internal/batching_publisher_connection_test.cc	99.62% <0.00%> (+0.18%)	⬆️
...integration_tests/schema_admin_integration_test.cc	100.00% <0.00%> (+1.11%)	⬆️

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 400ec14dff5ae32eafc73dd5d8034ca630c25817

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• Kokoro Logs (requires auth)

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