add method
Implementation
void add({required NsgDataItem item, DateTime? time, String? tag = ''}) {
if (item.primaryKeyField == '') return;
var id = item.getFieldValue(item.primaryKeyField).toString();
if (id == '') return;
time ??= DateTime.now();
items[id] = NsgDataCashedItem(dataItem: item, loadedTime: time, tag: tag);
}