Add tracks to playlist

Add an array of tracks to the playlist at {playlistId}. All metadata (e.g. track title, artist, etc) for tracks needs to be provided in the API call. The "source" property is optional, and defaults to 7digital. This relates to the audioUrl and image response and can be used to indicate a different source for the physical file, it can be any string you want as long as the application you use to add the track can understand it.The "audioUrl" property is optional and allows you to specify a different source for the physical file e.g. source="local" and audioUrl="file://a.specific.file.location.mp3" could mean that the consuming application understands how to handle a file stored at the given audioUrl.It is advisable that you only use these extra parameters if you need to use them. It's also advisable if you are accessing public playlists that some tracks may not be playable by your application, so we would advise including an option to hide, grey out non source=7digital tracks.