Change packaging to maven-plugin

[kubamarchwicki]

Maven plugin descriptor is not getting correctly generated, therefore example usage of openapi-diff with maven fails.
Changing the packaging fixes the issue.

Additional maven-example (a plugin verification) is also provided within this PR

[kubamarchwicki]

It ain't obvious to provide a test case to cover the change but I've added a minimal maven-plugin example which fails with the current setup: https://github.com/OpenAPITools/openapi-diff/compare/master...kubamarchwicki:maven-plugin-example?expand=1

If you feel it's relevant to the project, I'm happy to contribute it in a separate PR.

[joschi]

@kubamarchwicki Thanks a lot for your contribution!

I think the reproducer you've linked to would also be useful to avoid regressions. 😄

Do you want to add it in this pull request or create a new one?

[kubamarchwicki]

Do you want to add it in this pull request or create a new one?

I've added to this PR. I've linked reproducer separately as I wasn't sure if you want it in the main project :)