Enable FluentAPI to add dynamically response type
It's possible to add an overload in Fluent API to register also the response type together with the routing path?
For example: Routes.Add(requestType,restPath,verbs,responseType).
This can be useful when you can't use the IReturn Interface on DTO objects (for example inside UniversalApp PCL).

This defeats the primary purpose of the `IReturn` marker for enabling a terse strong-typed generic API on Service Clients which is only possible if the generic type information is statically defined.
You can use one of the other fallback options for specifying the ResponseType:
https://github.com/ServiceStack/ServiceStack/wiki/Metadata-page#matching-requests-with-their-response-dtos