[REQ] [dart] Generate a copyWith help method on models

Created by: jaumard

Is your feature request related to a problem? Please describe.

It can be very useful to generate a copyWith help method on the generated models. Generated models are immutable so to change a properties you need to recreate an object, a copyWith method will be super helpful for that.

Describe the solution you'd like

Modify the template to generate a copyWith method

Describe alternatives you've considered

Find a dart package that can generate this for us and just use it, but it will add an external dep so I don't recommend it.

What do you think guys ? @ircecho (2017/07) @swipesight (2018/09) @nickmeinhold (2019/09)