Gravity update Gui (backport #3197)

[mergify]

🎉 New Feature

Update Gravity with GUI

Summary

It adds the ability to change gravity in runtime using gazebo gui with its entity inspector

Test it

1. Open gazebo with gz sim shapes.sdf and hit play button to start the simulation
2. Try changing Gravity from the inspector and see that shapes will move accordingly

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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)
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This is an automatic backport of pull request #3197 done by Mergify.