[Dart-dio] Generator force using built_value

Created by: jaumard

Currently dart dio generator is using built_value to serialize/deserialize the json, would be nice to have more choice.

I'm thinking about freezed for example, because if we use this one in your app, you probably don't want to include built_value too...

Also a new feature will come soon to dart for meta programming, it will probably change the way we serialize/deserialize and maybe offer a full dart way of doing it. So preparing for multi serializer is probably a good thing