Allow enabling sand duping (#10191)

Because this exploit has been widely known for years and has not been fixed by Mojang, we decided that it was worth allowing people to toggle it on/off due to how easy it is to make it configurable.

It should be noted that this decision does not promise all future exploits will be configurable.

patches/server/0953-Allow-null-itemstack-for-Player-sendEquipmentChange.patch

@@ -0,0 +1,19 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: David Scandurra <david.scandurra@check24.de>
+Date: Wed, 25 Oct 2023 20:36:25 +0200
+Subject: [PATCH] Allow null itemstack for Player#sendEquipmentChange
+
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/entity/CraftPlayer.java b/src/main/java/org/bukkit/craftbukkit/entity/CraftPlayer.java
+index ff3db4efd2c10b3351453656937e88064267fb22..0b34a90d275534a84bea7d7d9cce08b41ea7a698 100644
+--- a/src/main/java/org/bukkit/craftbukkit/entity/CraftPlayer.java
++++ b/src/main/java/org/bukkit/craftbukkit/entity/CraftPlayer.java
+@@ -1047,7 +1047,7 @@ public class CraftPlayer extends CraftHumanEntity implements Player {
+ 
+     @Override
+     public void sendEquipmentChange(LivingEntity entity, EquipmentSlot slot, ItemStack item) {
+-        this.sendEquipmentChange(entity, Map.of(slot, item));
++        this.sendEquipmentChange(entity, java.util.Collections.singletonMap(slot, item)); // Paper - replace Map.of to allow null values
+     }
+ 
+     @Override