Publish step size in world stats topic

[iche033]

🎉 New feature

Summary

The step_size field in WorldStatistics msg is currently not populated / published in the /stats topic. This PR populates the field in a way that conforms to the msg documentation: /// \brief Iteration step size. It's zero when paused.

Test it

# run gz sim
gz sim -v 4 -r shapes.sdf

...

# echo the stats topic
gz topic -e -t /world/shapes/stats

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)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

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