Helper to get version number from package.xml

[scpeters]

🎉 New feature

Follow-up to https://github.com/gazebosim/gz-math/pull/639, part of https://github.com/gazebosim/sdformat/pull/1491

Summary

This adds a python script and a cmake helper function for getting the version number from a package.xml file.

Test it

Test using the version_from_package_xml example.

1. Build and install gz-cmake from this branch
2. cd examples/version_from_package_xml
3. mkdir build && cd build
4. cmake ..
5. You should see the following lines in the console output, pulling the version number from the example package.xml file

-- PACKAGE_XML_VERSION 8.21.65
-- PACKAGE_XML_VERSION_MAJOR 8
-- PACKAGE_XML_VERSION_MINOR 21
-- PACKAGE_XML_VERSION_PATCH 65
-- gz-version_from_package_xml version 8.21.65

Checklist

• [X] Signed all commits for DCO
• [X] Added tests
• [X] Added example and/or tutorial
• [X] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [X] codecheck passed (See contributing)
• [X] 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.