get method
Retrieve a CartItemModel with it's id, cached
Implementation
@override
Future<CartItemModel?> get(String? id, {Function(Exception)? onError}) async {
var value = fullCache[id];
if (value != null) return refreshRelations(value);
value = await reference.get(id, onError: onError);
fullCache[id] = value;
return value;
}