Support currently_playing_type=ad

[marcelstoer]

The SpotifyPlayingType enum does not currently support the ad value. You'd see this while ads are playing when on the free tier (i.e. non-premium account).

https://github.com/witnessmenow/spotify-api-arduino/blob/main/src/SpotifyArduino.h#L96C6-L101C3

The object type of the currently playing item. Can be one of track, episode, ad or unknown.

Source: https://developer.spotify.com/documentation/web-api/reference/get-the-users-currently-playing-track

I plan on submitting a MR for this.