Add option to disable building python bindings

[mjcarroll]

🦟 Bug fix

Addresses https://github.com/gazebosim/gz-cmake/issues/300

Summary

• Adds SKIP_PYBIND11 to allow for Python binding generation to be skipped.

Checklist

• [ ] Signed all commits for DCO
• [ ] Added tests
• [ ] 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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