This method is similar to ArrayableTrait::fields except that the list of fields returned
by this method are not returned by default by ArrayableTrait::toArray. Only when field names
to be expanded are explicitly specified when calling ArrayableTrait::toArray, will their values
be exported.
The default implementation returns an empty array.
You may override this method to return a list of expandable fields based on some context information
(e.g. the current application user).