PkPass diffing for change messages

[ueman]

When a PkPass file is updated, the system is supposed to show a change message notification/hint (depending on whether it was updated in background/foreground).
This is done by comparing the fields of the old and updated pass.

Those fields with a change message should be returned with a properly formatted change message. Otherwise, a generic message should be returned.

An open question is whether those fields support localization, but I would assume so.

We could provide an API that looks something like this:

class PkPass {
  List<String> getChangeMessages(PkPass oldPass, String locale) {
    // ...
  }
}

final changeMessages = newPass.getChangeMessages(oldPass, 'de_DE');