Currently, we allow changing an unstable field to stable or adding a stable field, but people could add/change them without fully understanding the implications of what adding a field to the stable API means (e.g. you can't ever remove that field from the stable API within this API version).

We should make it harder by creating a list similar to the existing ALLOW_ANY_TYPE_LIST that people have to add their fields to in order to allow them to be added as/changed to stable.