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: #181

[kdeldycke]

An improvement of this PR would be to use a enum.StrEnum class to reference all these types, so they can be used as constants.

enum.StrEnum being available starting with Python v3.11, we can add the optional StrEnum dependency for python_version < 3.11.

[mjherzog]

This PR needs to be reset to reflect the deprecation of https://github.com/package-url/purl-spec/blob/main/PURL-TYPES.rst
in favor of registered PURL types at: https://github.com/package-url/purl-spec/blob/main/types.
The file : https://github.com/package-url/purl-spec/blob/main/purl-types-index.json provides the current list of registered PURL types and is automatically regenerated whenever a new PURL type is added