Index builder does not recognize valid manifest entries

[awvwgk]

The index build currently warns for unknown entries in manifests like, categories, keywords, homepage and install. Those entries are valid as defined in the manifest reference in the fpm repository but not correctly recognized by the index builder.

[awvwgk]

I created a pydantic model of the fpm manifest a while ago in awvwgk/pyfpm, which allows to easily verify the correctness of a package manifest and extract the data in Python. Using this or a similar model would make it easy for the registry handle the package verification.