Add `PURL_TYPES` and enforce validation of PURL types

[kdeldycke]

This PR adds a comprehensive list of all pURL types that are referenced in https://github.com/package-url/purl-spec/blob/master/PURL-TYPES.rst

It also enforces the validation of a pURL type.

For context, I made this list for my Meta Package Manager project, which is mapping pURL types to package managers.

Closes: https://github.com/package-url/packageurl-python/issues/181