Add package.xml and validation workflow

[scpeters]

🎉 New feature

Adds a package.xml file

Summary

I noticed that our Gazebo metapackages don't have package.xml files, but they easily could, so I added one for gz-jetty with @azeey as the maintainer.

Test it

• Confirm that added workflow passes
• Add gz-jetty to a colcon workspace with other jetty packages and confirm that colcon graph properly recognizes package order

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

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.

[azeey]

I don't have any objection to adding package.xml file here, but I also don't know if there are any advantages for doing so.

[scpeters]

I don't have any objection to adding package.xml file here, but I also don't know if there are any advantages for doing so.

if we ever use bloom to generate packaging metadata from package.xml, it could be helpful. otherwise I think it just provides consistency with our other gz-* packages