Leverage plugin framework inside (test) catalog plugin  #55

The current repo killbill-catalog-plugin-test is not using this repo as a base.

We should leverage all the new api POJOs (see #54) and stop pulling the classes from the killbill repo, i.e remove this dependency.

However, such dependency is also used to deserialize the XML catalog, by leveraging all the JAXB annotation. Perhaps instead we could use as a source JSON catalog files (see https://killbill.github.io/slate/#catalog-retrieve-the-catalog-as-json to extract such catalog json), and from the json data write a json parser leveraging jackson and the new api POJOs from this repo.