Add support for retrofit. Object? with generics

[vanlooverenkoen]

@JsonSerializable(genericArgumentFactories: true)
class Response<T> {
  int status;
  T value;
}

will require 2 functions

  T Function(Object json) fromJsonT,
&
  Object Function(T value) toJsonT,

These 2 functions require an Object. -> retrofit is already compatible. They will generate the correct generic type. But the model generator will always use Map<String,dynamic>

We should have an option to generate something like this.

factory Project.fromJson(Object? json) => _$ProjectFromJson(json as Map<String, dynamic>);