Leaderboards only store a single numerical value per user as far as I'm aware, and it has to increment... With some bitwise abuse you could store a single 21 bit value, and save up to 4 billion times. While it's a fun abuse of the leaderboard system I really don't recommend this... For many reasons.
You probably want something like firebase: cloud firestore. Allows you to store and query application data from iOS, Android or the web. I've heard a few people talking about it on the forums before, not sure if there's a third party plugin going around for it or not.