Add immutable_fields global settings

[Nico04]

I have some large classes with a lot of fields, and I need to the copyWith for just one field.
Instead of decorate all my fields with @CopyWithField(immutable: true) but one, it would be very nice to have a global build.yaml setting (and/or a class-level tag) to define that all fields are by default immutable, exept the one annoted with @CopyWithField(immutable: false) ?