display_value support for custom field

[Ksnz]

API says that custom field has field named display_value

display_value is a read-only field that will always be a string. For apps that use custom fields, this is a great way to safely display/export the value of a custom field, regardless of its type. We suggest apps use this field in order to future-proof for changes to Custom Fields.

But com.asana.models.CustomField has no such field.