Add a ROS node that runs Gazebo

[azeey]

🎉 New feature

Closes #497

Summary

• [x] Add gzserver with ability to load an SDF file or string
• [ ] Add ability to load additional systems in addition to the ones loaded by default or as part of the SDF world
• [ ] Configure SDFormat to understand package:// URIs

Test it

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.