Provides step size via CLI interface and `ServerConfig`

[arjo129]

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🎉 New feature

Closes #<NUMBER>

Summary

This is probably not the best time to be making this PR as we are in the middle of the release. However for another project, I'd like to be able to set the Step Size via CLI. This is particularly useful for when we are doing integration testing of that project. This PR provides two command line flags that allow you to set the physics update rate and step size via cli. To try it out run:

gz sim shapes.sdf --physics-rtf 5.0 --physics-step-size 0.01

Note: Setting parameters via CLI/server_config should overwrite the world configs.

TODO: Update tests.

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.

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