feat(sql): add Backups and FeatureEligibility services

[scotthart]

PiperOrigin-RevId: 952134922

[gemini-code-assist]

Code Review

This pull request expands the Cloud SQL Admin API library to include the Backups and FeatureEligibility services, which are configured to use REST transport only. The review feedback correctly points out that the documentation in environment-variables.dox incorrectly references the non-existent gRPC connection factories MakeSqlBackupsServiceConnection() and MakeSqlFeatureEligibilityServiceConnection(), and should be updated to refer to their REST counterparts (MakeSqlBackupsServiceConnectionRest() and MakeSqlFeatureEligibilityServiceConnectionRest()).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.30%. Comparing base (842dfb2) to head (53ed804).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16302      +/-   ##
==========================================
- Coverage   92.30%   92.30%   -0.01%     
==========================================
  Files        2223     2223              
  Lines      208177   208177              
==========================================
- Hits       192165   192158       -7     
- Misses      16012    16019       +7     

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