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 (d7d1c16) to head (ef9dc4f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16302   +/-   ##
=======================================
  Coverage   92.30%   92.30%           
=======================================
  Files        2226     2226           
  Lines      208573   208573           
=======================================
+ Hits       192517   192519    +2     
+ Misses      16056    16054    -2     

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