Feature: Support Union in DjangoFilterConnectionField and DjangoConnectionField

[that-one-arab]

This issue was previously mentioned here

The purpose of this PR is to make Union types compatible with DjangoFilterConnectionFields so that we can query a root model that is resolved to other model types in runtime, supporting libraries like django-polymorphic

Here's an example of what this PR achieves:

    class AssessmentUnion(DjangoUnionType):
        class Meta:
            model = Assessment
            types = (HomeworkAssessmentNode, QuizAssessmentNode)
            interfaces = (graphene.relay.Node,)
            filter_fields = ("id", "title", "description")
        @classmethod
        def resolve_type(cls, instance, info):
            if isinstance(instance, HomeworkAssessment):
                return HomeworkAssessmentNode
            elif isinstance(instance, QuizAssessment):
                return QuizAssessmentNode


    class Query(graphene.ObjectType):
        all_assessments = DjangoFilterConnectionField(AssessmentUnion)       

Previously, DjangoFilterConnectionField only accepted DjangoObjectTypes and DjangoObjectTypes do not support unions.

Has this PR dependancy that needs merging for the feature to work

NOTE: Tests are failing because it depends on the aforementioned PR!

[that-one-arab]

@ulgens I made the changes as requested, can you please re-review when you can?

[that-one-arab]

@ulgens @jaw9c @kiendang @mahdyhamad What are your opinions on this PR? Should I close it?