provide facilities to make it easier for users to use paginated APIs

Some APIs support pagination - being able to specify the max. number of items returned, and being able to pass in a `nextPageToken` to page through the results. We should make it easier for users to work with these types of APIs. That might look like something like generating a 2nd method with a stream of results, and having that method automatically handle re-querying the backend for more results.

```dart
Future listItems(...) {
...
}

Stream listItemsStreaming(...) {
...
}
```