Allow the pageBuilder to provide the amount of total items

[hacker1024]

This should be possible so the loading widget isn't shown at the end of the list.

[hacker1024]

At the moment, the following workaround is an option:

int _totalItems;

...

Pagination<Data>(
  pageBuilder: (currentSize) async {
    if (currentSize == _totalItems) return const [];
    final data = await getData(currentSize);
    _totalItems = data.totalItems;
    return data;
  },
  itemBuilder: ...
);