Standalone gz sim executable

[sauk2]

🎉 New feature

Related to gazebosim/gz-tools#7

Summary

This PR introduces a standalone executable for gz sim by following a similar approach to gz model. The new executable is integrated into cmdsim.rb.in, replacing the existing functionality.

The majority of the behavior remains consistent, including:

• Launching the Server using -s
• Launching the GUI using -g
• Launching the Quickstart menu when no argument is provided

Changes to the launch behaviour

• Launching with -s runs the server in the main thread.
• Launching with -g runs the GUI in the main thread.
• Running without -s or -g launches both the Server and GUI in separate threads. Since a single SIGINT terminates only one of the threads while the other continues running, a force kill mechanism ensures that both threads terminate properly.

Test it

Check for UNIT_gz_TEST and INTEGRATION_log_system.

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

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