Custom DefaultResolveFn

Hey,

I have a custom type system and i have few constraints:

1. I cannot implement Resolve on the object level as I cannot change the type system.
2. I don't want to implement a resolver on every low level field just to resolve the custom object
3. Default DefaultResolveFn fails to resolve the custom object.

Unless there's a better approach, I would like to provide a custom `DefaultResolveFn`.

Thanks.