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.Rest/API/Common/Game.cs

@@ -1,4 +1,4 @@
-#pragma warning disable CS1591
+#pragma warning disable CS1591
 using Newtonsoft.Json;
 using Newtonsoft.Json.Serialization;
 using System.Runtime.Serialization;
@@ -29,6 +29,10 @@ namespace Discord.API
         public Optional<API.GameTimestamps> Timestamps { get; set; }
         [JsonProperty("instance")]
         public Optional<bool> Instance { get; set; }
+        [JsonProperty("sync_id")]
+        public Optional<string> SyncId { get; set; }
+        [JsonProperty("session_id")]
+        public Optional<string> SessionId { get; set; }
 
         [OnError]
         internal void OnError(StreamingContext context, ErrorContext errorContext)