feat: add Django PackageURL field

[Elkasitu]

In some applications, PURLs can be used to identify / track certain types of models instead of being a model of its own.

In such cases, having the PURL as a field rather than a related entity can be useful and more convenient, especially when there's no need to track or edit individual PURL components from the backend.

I can provide unit tests but I'm not sure how to add test dependencies to the project.