add: integrate endpoint for fetching timezones

[daniele-mng]

What

Integrate the timezone-fetching endpoint into GSA settings so the UI can load available timezones.
Update the settings flow to consume the endpoint response as a simple list of timezone names
Add a failover mechanism using a static timezone list if the endpoint request fails.

Why

The settings UI requires a reliable source of valid timezone identifiers to populate the timezone selector and ensure consistent behavior.

References

GEA-1573

Checklist

• Tests

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Snapshot Warnings

⚠️: No snapshots were found for the head SHA f30b3e4.

Ensure that dependencies are being submitted on PR branches and consider enabling retry-on-snapshot-warnings. See the documentation for more information and troubleshooting advice.

Scanned Files

None

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.06%. Comparing base (bc931f8) to head (f30b3e4).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5136      +/-   ##
==========================================
+ Coverage   77.04%   77.06%   +0.01%     
==========================================
  Files        1170     1172       +2     
  Lines       30492    30512      +20     
  Branches     9084     9089       +5     
==========================================
+ Hits        23494    23515      +21     
+ Misses       6258     6257       -1     
  Partials      740      740              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.