Add URL to the vulnerability and package details view in the API serializers

[TG1999]

Reference: #1419

[tdruez]

• Use a common naming convention instead of package_url and vulnerability_url
• The URL build logic should live on the model for a global usage, not on the API, see https://docs.djangoproject.com/en/5.0/ref/models/instances/#get-absolute-url

[TG1999]

@tdruez we already have get_absolute_url method on both Package and Vulnerability models, but it returns /packages/pkg:foo/bar and not public.vulnerablecode.io/packages/pkg:foo/bar, I have added a new method get_details_url for the needed purpose. Please review, thanks!

[tdruez]

Your implementation has too much duplication.
When copy/pasting things around in Python, it's usually time for refactoring.

You can create a generic mixin including:

    def get_resource_url(self, instance):
        """
        Return the instance fully qualified URL including the schema and domain.

        Usage:
            resource_url = serializers.SerializerMethodField()
        """
        resource_url = obj.get_absolute_url()

        if request := self.context.get("request", None):
            return request.build_absolute_uri(location=resource_url)

        return resource_url

Then simply define a clean field resource_url = serializers.SerializerMethodField() on your serializers instead of overriding the to_representation

Also, the tests are failing.

[tdruez]

@TG1999 LGTM but the documentation check is failing. Ready to merge otherwise.