Add support for custom interfaces

[janbaykara]

Allow custom resolvers within a wagtail-grapple setup, by injecting an interface.

class Event(HeadlessPreviewMixin, Page):

    @classmethod
    def get_interface(self, interface):
        class EventInterface(graphene.Interface):
            saved = graphene.Boolean(argument_1=graphene.Boolean(required=True), required=True)

            def resolve_saved(self, info, **kwargs):
                print("resolve_saved", info.context.user)
                return False

        return (interface, EventInterface, )