We make no guarantees for type incompatibility for unstable fields, so we should allow for incompatible changes to unstable fields.

The one exception is that we should error if an unstable field is added with a new use of bson_serialization_type "any" without being added to the allowList. This is because unstable fields can be used for experimental fields that ultimately become stable, so we should warn away from using the "any" type as soon as we can.