Use a JSON to Java mapper

[roguib]

In many places of the codebase there's code that converts a plain string into a object:

final String jsonEvalRes = response.as(String.class);
JsonObject object = jsonReader.readObject();
jsonReader.close();
final EvalResponse evalRes = new EvalResponse(object.getString("result"), object.getString("status"));

This should be handled automatically by a mapper.