…ypes

This allows for using `Future<Response<anything>>` as return type for a method
Which allows for
things like `Future<Response<Container<SubType>>>` e.g. `Future<Response<BuiltList<String>>>`
This
is neccessary because otherwise the code generator outputs `Response<SubType>> instead of
`Response<Container<SubType>>`

…ters if it's null

The Dart's type system can infer these types from the super class

…n constructor

Since base url is a compile-time constant, we can do the validation during code generation

…eters

GitHub: @devkabiir