Refactor: deserialize response into internal types directly
The current approach uses the already deserialized types provided by prometheus-http-query and converting the outer types into internal types.
The plan is to do the deserialization directly, and output returned by the query method will only include the internal types.
This will unblock #13 (closed)
This will serve as a foundation for removing the need of using separate utils to convert labels to custom defined structs.