Expose decorator types

Expose decorator types like `ReturnTypeFuncValue`. This is useful if you want to implement a generic paginated response type. We use an abstract class aka `interface` for `itemsFieldValue`. An abstract doesn't fulfill `ClassType`. `ReturnTypeFuncValue` would allow us to use the same type as the decorator expects.

```ts
export default function PaginatedResponse(
itemsFieldValue: ClassType | String | Number | Boolean,
) {
// `isAbstract` decorator option is mandatory to prevent registering in schema
@ObjectType({ isAbstract: true })
abstract class PaginatedResponseClass {
@Field(type => [itemsFieldValue])
items: TItemsFieldValue[];

@Field(type => Int)
total: number;

@Field()
hasMore: boolean;
}
return PaginatedResponseClass;
}
```