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.Core/Entities/Activities/Game.cs

@@ -1,4 +1,4 @@
-using System.Diagnostics;
+using System.Diagnostics;
 
 namespace Discord
 {

src/Discord.Net.Core/Entities/Activities/GameAsset.cs

@@ -1,15 +1,15 @@
-namespace Discord
+namespace Discord
 {
     public class GameAsset
     {
         internal GameAsset() { }
 
-        internal ulong ApplicationId { get; set; }
+        internal ulong? ApplicationId { get; set; }
         
         public string Text { get; internal set; }
         public string ImageId { get; internal set; }
         
         public string GetImageUrl(ImageFormat format = ImageFormat.Auto, ushort size = 128)
-            => CDN.GetRichAssetUrl(ApplicationId, ImageId, size, format);
+            => ApplicationId.HasValue ? CDN.GetRichAssetUrl(ApplicationId.Value, ImageId, size, format) : null;
     }
-}
+}

src/Discord.Net.Core/Entities/Activities/RichGame.cs

@@ -1,4 +1,4 @@
-using System.Diagnostics;
+using System.Diagnostics;
 
 namespace Discord
 {
@@ -7,8 +7,8 @@ namespace Discord
     {
         internal RichGame() { }
 
-        public string Details { get; internal set;}
-        public string State { get; internal set;}
+        public string Details { get; internal set; }
+        public string State { get; internal set; }
         public ulong ApplicationId { get; internal set; }
         public GameAsset SmallAsset { get; internal set; }
         public GameAsset LargeAsset { get; internal set; }
@@ -19,4 +19,4 @@ namespace Discord
         public override string ToString() => Name;
         private string DebuggerDisplay => $"{Name} (Rich)";
     }
-}
+}

src/Discord.Net.Core/Entities/Activities/SpotifyGame.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+
+namespace Discord
+{
+    [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
+    public class SpotifyGame : Game
+    {
+        public string[] Artists { get; internal set; }
+        public string AlbumArt { get; internal set; }
+        public string AlbumTitle { get; internal set; }
+        public string TrackTitle { get; internal set; }
+        public string SyncId { get; internal set; }
+        public string SessionId { get; internal set; }
+        public TimeSpan? Duration { get; internal set; }
+
+        internal SpotifyGame() { }
+
+        public override string ToString() => Name;
+        private string DebuggerDisplay => $"{Name} (Spotify)";
+    }
+}