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

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