Support explicit init_after constraints so that fields can be initialized in another order than the default

[smarie]

The initialization order can be by default the order of appearance in the class. We could also add a init_after=<fields> argument to field() so that a field requiring other fields to be set before validation, converter or default_factory execution could be initialized afterwards. Another option would be to support a special way to declare that one of these three arguments depends on another field.

[smarie]

The workaround is to define the fields in the desired order in the class: indeed pyfields preserves this order of appearance (even in old versions of python, yay!)