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/0524-Add-Block-isValidTool.patch

@@ -0,0 +1,20 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Jake Potrebic <jake.m.potrebic@gmail.com>
+Date: Mon, 6 Jul 2020 12:44:31 -0700
+Subject: [PATCH] Add Block#isValidTool
+
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/block/CraftBlock.java b/src/main/java/org/bukkit/craftbukkit/block/CraftBlock.java
+index 98e87dc15e2ed23f6897ba6359846ff5bc32b655..2034858a53c4c887da334cdc7713997daa01124f 100644
+--- a/src/main/java/org/bukkit/craftbukkit/block/CraftBlock.java
++++ b/src/main/java/org/bukkit/craftbukkit/block/CraftBlock.java
+@@ -693,5 +693,9 @@ public class CraftBlock implements Block {
+     public String translationKey() {
+         return this.getNMS().getBlock().getDescriptionId();
+     }
++
++    public boolean isValidTool(ItemStack itemStack) {
++        return getDrops(itemStack).size() != 0;
++    }
+     // Paper end
+ }