Wandering Trader Restocking

Adds ability to mark one item (for free or maybe with a small emerald fee) so that the wandering trader will have that item in its sell list the next time it spawns. Only one item can be marked at a time. This would make certain optional challenges (such as survival superflat) easier while not overturning the wandering trader into giving you any item you want, while still requiring you to get lucky with the trader spawning with the desired item to be marked. This would likely require changing the spawning mechanics of the trader in multiplayer, but it would really make the game feel more alive and make the trader more enjoyable to encounter overall.