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.WebSocket/Extensions/EntityExtensions.cs

@@ -1,3 +1,5 @@
+using Discord.Rest;
+using System;
 using System.Collections.Immutable;
 using System.Linq;
 
@@ -7,6 +9,19 @@ namespace Discord.WebSocket
     {
         public static IActivity ToEntity(this API.Game model)
         {
+            // Custom Status Game
+            if (model.Id.IsSpecified)
+            {
+                return new CustomStatusGame()
+                {
+                    Type = ActivityType.CustomStatus,
+                    Name = model.Name,
+                    State = model.State.IsSpecified ? model.State.Value : null,
+                    Emote = model.Emoji.IsSpecified ? model.Emoji.Value.ToIEmote() : null,
+                    CreatedAt = DateTimeOffset.FromUnixTimeMilliseconds(model.CreatedAt.Value),
+                };
+            }
+
             // Spotify Game
             if (model.SyncId.IsSpecified)
             {