Support reference custom fields #46

Asana has a reference custom field type that links a task to other tasks. Its value is set by passing an array of object GIDs. asana-go has no FieldTypeReference constant and no ReferenceValue on CustomFieldValue or CustomField.

Proposal

• Add FieldTypeReference FieldType = "reference".
• Add ReferenceValue []*Task (compact) to CustomFieldValue.
• Check that creating a field with resource_subtype: reference works through CreateCustomField.

Ditto needs this to copy reference fields.

Raised by Claude on behalf of Mike. It comes from the Ditto vs Asana features comparison.