Resolve resource type in serializer's meta class

utils.get_resource_type_from_serializer is called several times throughout the DJA code base. When this method raises an AttributeError it is a configuration/code setup issue and not a runtime issue. Besides there is no error message describing want went wrong.

Goals:

• move logic of get_resource_type_from_serializer to serializer’s meta class
• improve error handling (better error message).

[SafaAlfulaij]

While we are at this, what do you think if we allowed serializers themselves to define what kind of type they are, via a function? This would overwrite the attribute access. Something very similar to what DRF does with queryset/get_queryset and serializer_class/get_serializer_class in views. This would help for polymorphic relations, generic relations and custom non-ORM relations and serializers.

[SafaAlfulaij]

I’m also thinking if we make included_serializers and included_resources the same way. This also would help polymorphic relations, generic relations and custom non-ORM relations and serializers.