Allow using NestedHyperlinkedRelatedField without a model-related field

[rmosquera-bjumper]

With this small change we can use the NestedHyperlinkedRelatedField with models that doesn't has a relation field between them. I don't know how to create a test demonstrating the use case; any help is welcome.

For example, in this case:

class Item:
  name = models.CharField(max_length=10)

class Doc:
  name = models.CharField(max_length=10)

class Attachment:
  item = models.ForeignKey(Item, on_delete=models.CASCADE)
  doc =  models.ForeignKey(Item, on_delete=models.CASCADE)

If we want to have an URL like /item/1/doc/123, we can do that with:

class ItemDocumentsViewSet(
    mixins.ListModelMixin,
    mixins.CreateModelMixin,
    mixins.DestroyModelMixin,
    viewsets.GenericViewSet,
):
    serializer_class = ItemDocumentSerializer

    def __init__(self, **kwargs):
        self.item_id = None
        super().__init__(**kwargs)

    def initialize_request(self, request, *args, **kwargs):
        request = super().initialize_request(request, *args, **kwargs)
        self.item_id = kwargs.get("item_id")
        return request

   # ....

class ItemDocumentSerializer(HyperlinkedModelSerializer):
    url = NestedHyperlinkedIdentityField(
        view_name="item-documents-detail",
        lookup_url_kwarg="id",
        parent_lookup_kwargs={"item_id": "item_id"},
    )
    
    class Meta:
        model = Doc
        fields = ("url", "name")