Add support for inheritance when reading derived fields

[georg-getz]

You should be able to have the option to pass an interface as the type of a derived field in the schema.graphql examples: [IExample!]! @derivedFrom(field: "main") If IExample in the code above is an interface the whole field is going to be ignored because when the derived hashmap is created it would put IExample as the entity type to look for when store.set() is called whereas it should be looking for any entities that implement it.