Expose oauth access token via api interface

osu.Game/Online/API/IAPIProvider.cs

@@ -19,6 +19,11 @@ namespace osu.Game.Online.API
         /// </summary>
         Bindable<UserActivity> Activity { get; }
 
+        /// <summary>
+        /// Retrieve the OAuth access token.
+        /// </summary>
+        public string AccessToken { get; }
+
         /// <summary>
         /// Returns whether the local user is logged in.
         /// </summary>