Support `episode`s when retrieving items from playlist

[hayribakici]

So far only tracks are retrieved from getTracksByPlaylistId. Spotify however also returns episodes.

[hayribakici]

This kind of breaks the current Pages architecture. Currently, each Page can only hold one type with its dedicated parser. With multiple types, you need multiple parsers. So far, I am able to solve this issue with extending the BasePage and Page by another constructor and adding a new class MultiPage that allows multiple types:

  BasePage(this._paging, ParserFunction<T> pageItemParser,
      [Object? pageContainer]) {
    _items = _paging.itemsNative!.map(pageItemParser);
    _container = pageContainer;
  }


  BasePage.fromParserMap(
      this._paging, Map<String, ParserFunction<dynamic>> parserMap,
      [Object? pageContainer]) {
    _items = _paging.itemsNative!.map((item) {
      // so according to the spotify documentation, 
      // a json key `track` can either be a `Track` or `Episode` object.
      // Distinguishing between the two types is therefore only
      // possible by checking their `type` and therefore
      // run the according parser through that json object.
      var type = item["track"]["type"] as String; // <-- this is the line I don't like, needs further discussion
      if (parserMap.containsKey(type)) {
        return parserMap[type]!(item);
      }
      throw TypeError();
    });
    _container = pageContainer;
  }

...
class Page ... {
Page.fromParserMap(
      Paging<T> paging, Map<String, ParserFunction<dynamic>> parserMap,
      [Object? pageContainer])
      : super.fromParserMap(paging, parserMap);

}
...

/// Handles retrieval of a page with multiple types
class MultiPage extends SinglePages<dynamic, Page<dynamic>> with OffsetStrategy<Page<dynamic>> {
  final Map<String, ParserFunction<dynamic>> _pageMappers;

  MultiPage(SpotifyApiBase api, String path, this._pageMappers)
      : super(api, path, null, null);

  @override
  Future<Page<dynamic>> getPage(int limit, [int offset = 0]) async {
    var pathDelimiter = _path.contains('?') ? '&' : '?';
    var newPath = '$_path${pathDelimiter}limit=$limit&offset=$offset';

    var jsonString = await _api._get(newPath);
    var paging = Paging.fromJson(json.decode(jsonString));

    return Page.fromParserMap(paging, _pageMappers);
  }
}

And finally, distinguishing types in this new MultiPage class should be done with runtimeType.

var items = spotify.playlists.getItemsByPlaylistId('...');
var page = (await items.first()).items;
expect(page?.first?.runtimeType, Track);

I marked the line, where we could discuss this a bit more @rinukkusu .