Add featured artist track ID online info

osu.Game/Beatmaps/BeatmapSetOnlineInfo.cs

@@ -90,6 +90,12 @@ namespace osu.Game.Beatmaps
         /// The song language of this beatmap set.
         /// </summary>
         public BeatmapSetOnlineLanguage Language { get; set; }
+
+        /// <summary>
+        /// The song ID of this beatmap set.
+        /// Non-null only if the song is from a featured artist.
+        /// </summary>
+        public int? TrackId { get; set; }
     }
 
     public class BeatmapSetOnlineGenre

osu.Game/Online/API/Requests/Responses/APIBeatmapSet.cs

@@ -63,6 +63,9 @@ namespace osu.Game.Online.API.Requests.Responses
         [JsonProperty(@"ratings")]
         private int[] ratings { get; set; }
 
+        [JsonProperty(@"track_id")]
+        private int? trackId { get; set; }
+
         [JsonProperty(@"user_id")]
         private int creatorId
         {
@@ -106,7 +109,8 @@ namespace osu.Game.Online.API.Requests.Responses
                     Availability = availability,
                     HasFavourited = hasFavourited,
                     Genre = genre,
-                    Language = language
+                    Language = language,
+                    TrackId = trackId
                 },
             };