Add a PurlTypes enum

[lmco-jeaboswell]

I am using the packageurl-python library, and I need to be able to iterate through all of the supported purl types.

I am currently creating my own, but I think it would be a great addition to have it part of the library.

[giterlizzi]

You can use this list, which contains all officially supported PURL types.

https://github.com/package-url/purl-spec/blob/main/purl-types-index.json

[lmco-jeaboswell]

Yeah, that is what I am using to make my Enum, but I don't see it included in the actual python package. Did I miss it somewhere?