Add support for coupons to be limited to LuckPerms groups alongside usernames

At the moment, coupons can only be limited to permit usernames / UUIDs and while this works for limiting coupons to single players / members, this doesn’t work if you’d like to give a certain group of members (like server staff for example) a discount on their purchases in your store.

On the assumption it’s possible, I’d like to see a feature for limiting the use of coupons to LuckPerms groups alongside the existing usernames / UUIDs feature.

This would allow the coupon to only be used by those in a specific LuckPerms group or groups for example, either a single group or multiple groups could be allowed (for example: “administrator” or “administrator,developer,management”).

This would also ensure that the coupon can’t be used by those that are removed from a specified group or groups so the coupon can’t be shared for use by other players / members.