Method GetPlayerRank
GetPlayerRank(string, string, Action<int, int>, Action<Exception>)
Returns the current player's 1-based rank and score via onSuccess.
Rank is computed using two server-side aggregate counts: players with a strictly higher score,
and players with the same score but an earlier submission timestamp (tie-breaking).
public static void GetPlayerRank(string leaderboardName, string scoreName, Action<int, int> onSuccess, Action<Exception> onError = null)