Example of using the static plugin in gz-sim

[mjcarroll]

🎉 New feature

Related to: https://github.com/gazebosim/gz-plugin/pull/97

Summary

A quick demo of how to use statically linked plugins inside of gz-sim.

Test it

• The INTEGRATION_reset_sensors test should still work without using the shared library.
• The UNIT_system_loader should show additional statically linked plugins

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.

[chapulina]

Since this PR isn't adding a new feature, but just adding a test / demo for an existing one, I swapped the enhancement label for bug, added tests and documentation, and retargeted the PR to the new gz-sim7 branch so it could be merged until code freeze.