Gravity set command fixed (backport #3189)

[mergify]

🦟 Bug fix

Summary

This pull requests solves the issue in gz-physics but since it requires changes in this repo instead of gz-physics I am opening this pr here.
With this gazebo now supports changing gravity at runtime by running
'gz service -s /world/world_demo/set_physics --reqtype gz.msgs.Physics --reptype gz.msgs.Boolean --timeout 2000 --req 'gravity { x: 0 y: 0 z: -2.8 }'

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers
• Was GenAI used to generate this PR? If so, make sure to add "Generated-by" to your commits. (See this policy for more info.)

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by and Generated-by messages.

Signed-off-by: Shivang [email protected]

---

This is an automatic backport of pull request #3189 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.00%. Comparing base (b682e0a) to head (e0298fc).
⚠️ Report is 19 commits behind head on gz-sim8.

Additional details and impacted files

@@           Coverage Diff            @@
##           gz-sim8    #3191   +/-   ##
========================================
  Coverage    68.99%   69.00%           
========================================
  Files          342      346    +4     
  Lines        33432    33439    +7     
========================================
+ Hits         23068    23076    +8     
+ Misses       10364    10363    -1     

☔ 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.