WIP: Implement gz sdf as standalone executable

[scpeters]

🎉 New feature

Part of gazebosim/gz-tools#7.

Summary

This is a work-in-progress to implement the gz sdf functionality in a standalone executable (similar to gazebosim/gz-transport#216). The standalone executable has added alongside the existing approach. Once the standalone executable is complete and functioning, the old approach should be removed.

This is not targeting the first release of Gazebo Ionic.

Test it

Use UNIT_gz_TEST as the guide.

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.