appendPage method
Append new items to the current list and set the next page key
Implementation
@override
void appendPage(List<T> newItems, PageKeyType? nextPageKey) {
final updatedItems = <T>[...(state.itemList ?? []), ...newItems];
state = state.copyWith(
itemList: updatedItems,
nextPageKey: nextPageKey,
error: null,
);
}