support interface retrieve

[conanchen]

Is your feature request related to a problem? Please describe.
How to retrieve "Node" with graphqlWebClient?
I am trying to access Node with an ID :

1. graphqlWebClient.post(query, Node.class)
2. Node.java

public interface Node {
    String getGraphQlTypeName();

    ID getId();
}

but got error message:
Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of xxx.graphql.generated.Node(no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information

[oliemansm]

This library relies on Jackson for deserialization as the error message you've pasted here indicates. Node needs to be a class compatible with Jackson and not an interface like it is now. Try Googling how to use Jackson and the error message itself to find more information how Jackson works.