Java generator: Union type handling

[sonallux]

With the addition of Episodes and Shows, the Spotify Web API does use union types (for example can Playlists can contain Episodes and/or Tracks). As Java does not natively support unions, any union type is currently mapped to Map<String, Object>, which is not a nice UX.

Once shared super types are added (see #7) the handling of union types in the Java wrapper can be greatly improved.
For example the getPlaylistTrack() method can then be typed with a shared super type:

Paging<SpotifyObject> getPlaylistTracks(...) {
}

The user can then inspect the type property of the SpotifyObject and cast to either Track or Episode. This would require a custom Jackson deserializer that inspects the type property and then creates an instance of the concrete SpotifyObject class (e.g. Artist, Album, Track, Playlist, Show, Episode).