Package create macros for garden

[harshmahesheka]

🎉 New feature

Implementation of https://github.com/gazebosim/gz-cmake/pull/262 for Garden

Summary

The idea is to create a series of macros that will help new and old users create new packages quickly. This pr changes ign to gz and other small changes to make macors compatible for the garden.

Checklist

• [x] 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.