Split out a couple more nested classes

osu.Game/Online/API/Requests/Responses/APIUserAchievement.cs

@@ -0,0 +1,17 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using Newtonsoft.Json;
+
+namespace osu.Game.Online.API.Requests.Responses
+{
+    public class APIUserAchievement
+    {
+        [JsonProperty("achievement_id")]
+        public int ID;
+
+        [JsonProperty("achieved_at")]
+        public DateTimeOffset AchievedAt;
+    }
+}