Feature: CustomStatusGame Activity (#1406)

* Implement CustomStatusGame activity

Adds the CustomStatusGame class, which is the activity corresponding to the custom status feature.

* Remove unused import from Game.cs

src/Discord.Net.Rest/API/Common/Game.cs

@@ -35,6 +35,12 @@ namespace Discord.API
         public Optional<string> SessionId { get; set; }
         [JsonProperty("Flags")]
         public Optional<ActivityProperties> Flags { get; set; }
+        [JsonProperty("id")]
+        public Optional<string> Id { get; set; }
+        [JsonProperty("emoji")]
+        public Optional<Emoji> Emoji { get; set; }
+        [JsonProperty("created_at")]
+        public Optional<long> CreatedAt { get; set; }
 
         [OnError]
         internal void OnError(StreamingContext context, ErrorContext errorContext)