Add Spotify track support (#970)

* Initial Spotify support

* Remove GameAsset#ToEntity

- appId doesn't seem to be necessary, and Spotify Game doesn't return appId either.

* Implement SpotifyGame details

* Implement song Duration prop

* Add album art CDN

* Fix ActivityType

* Remove payload debug

* Add changes according to review

+ Make `ApplicationId` nullable
+ Move ctor after props

src/Discord.Net.WebSocket/Extensions/EntityExtensions.cs

@@ -4,6 +4,27 @@ namespace Discord.WebSocket
     {
         public static IActivity ToEntity(this API.Game model)
         {
+            // Spotify Game
+            if (model.SyncId.IsSpecified)
+            {
+                var assets = model.Assets.GetValueOrDefault()?.ToEntity();
+                string albumText = assets?[1]?.Text;
+                string albumArtId = assets?[1]?.ImageId?.Replace("spotify:","");
+                var timestamps = model.Timestamps.IsSpecified ? model.Timestamps.Value.ToEntity() : null;
+                return new SpotifyGame
+                {
+                    Name = model.Name,
+                    SessionId = model.SessionId.GetValueOrDefault(),
+                    SyncId = model.SyncId.Value,
+                    AlbumTitle = albumText,
+                    TrackTitle = model.Details.GetValueOrDefault(),
+                    Artists = model.State.GetValueOrDefault()?.Split(';'),
+                    Duration = timestamps?.End - timestamps?.Start,
+                    AlbumArt = albumArtId != null ? $"https://i.scdn.co/image/{albumArtId}" : null,
+                    Type = ActivityType.Listening
+                };
+            }
+
             // Rich Game
             if (model.ApplicationId.IsSpecified)
             {
@@ -34,7 +55,7 @@ namespace Discord.WebSocket
         }
 
         // (Small, Large)
-        public static GameAsset[] ToEntity(this API.GameAssets model, ulong appId)
+        public static GameAsset[] ToEntity(this API.GameAssets model, ulong? appId = null)
         {
             return new GameAsset[]
             {