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/0649-Prevent-softlocked-end-exit-portal-generation.patch

@@ -0,0 +1,22 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Noah van der Aa <ndvdaa@gmail.com>
+Date: Mon, 30 Aug 2021 15:22:18 +0200
+Subject: [PATCH] Prevent softlocked end exit portal generation
+
+
+diff --git a/src/main/java/net/minecraft/world/level/dimension/end/EndDragonFight.java b/src/main/java/net/minecraft/world/level/dimension/end/EndDragonFight.java
+index 1c9bfcf595bcb247c3b9ffe8b0e0cfee0d0ba91f..50071bec6825ec2b34662de14128070510d16afb 100644
+--- a/src/main/java/net/minecraft/world/level/dimension/end/EndDragonFight.java
++++ b/src/main/java/net/minecraft/world/level/dimension/end/EndDragonFight.java
+@@ -466,6 +466,11 @@ public class EndDragonFight {
+             }
+         }
+ 
++        // Paper start - Prevent "softlocked" exit portal generation
++        if (this.portalLocation.getY() <= this.level.getMinBuildHeight()) {
++            this.portalLocation = this.portalLocation.atY(this.level.getMinBuildHeight() + 1);
++        }
++        // Paper end - Prevent "softlocked" exit portal generation
+         if (worldgenendtrophy.place(FeatureConfiguration.NONE, this.level, this.level.getChunkSource().getGenerator(), RandomSource.create(), this.portalLocation)) {
+             int i = Mth.positiveCeilDiv(4, 16);
+