/// <summary>
 /// Link an user's id from an Identity Provider to the main user.
 /// </summary>
 /// <param name="info">This is the Identity Provider information to link to the main user.</param>
 /// <param name="callback">The callback after the async operation is done.</param>
 public abstract void Link(ExternalToken info, Callback callback);
 /// <summary>
 /// Login/Create the user using external token.
 /// </summary>
 /// <param name="externalToken"></param>
 /// <param name="callback">The callback after the async operation is done.</param>
 public abstract void ExternalAuth(ExternalToken externalToken, Callback callback);