Standalone gz model executable

[sauk2]

🎉 New feature

Part of gazebosim/gz-tools#7

Summary

This PR introduces a standalone gz model executable, following a similar approach to gz-transport. The new executable is integrated into cmdmodel.rb.in, replacing its existing functionality.

The gz sim implementation remains unchanged in this PR and will be addressed in a subsequent update. Additionally, the command-line tests (gz_TEST.cc and ModelCommandAPI_TEST.cc) are now configured to build after the gz model executable is created.

Test it

Check for UNIT_gz_TEST and UNIT_ModelCommandAPI_TEST

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)
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