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/0467-Optimize-Dynamic-get-Missing-Keys.patch

@@ -0,0 +1,34 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Mon, 21 Dec 2020 11:01:42 -0500
+Subject: [PATCH] Optimize Dynamic#get Missing Keys
+
+get was calling toString() on every NBT object that was ever asked for an optional
+key from the object to build a string for the error text.
+
+When done on large NBT objects, this was using a ton of computation time building the
+JSON representation of the NBT object.
+
+Now we will just skip the value when 99.9999% of the time the text is never even printed.
+
+diff --git a/src/main/java/com/mojang/serialization/Dynamic.java b/src/main/java/com/mojang/serialization/Dynamic.java
+index a24cc02a62585d7c04d36456739b11bddccf1a15..984e41619230db4579b028af18670c471aa13096 100644
+--- a/src/main/java/com/mojang/serialization/Dynamic.java
++++ b/src/main/java/com/mojang/serialization/Dynamic.java
+@@ -17,6 +17,7 @@ import java.util.stream.Stream;
+ 
+ @SuppressWarnings("unused")
+ public class Dynamic<T> extends DynamicLike<T> {
++    private static final boolean DEBUG_MISSING_KEYS = Boolean.getBoolean("Paper.debugDynamicMissingKeys"); // Paper - Perf: Skip toString on values like NBT
+     private final T value;
+ 
+     public Dynamic(final DynamicOps<T> ops) {
+@@ -113,7 +114,7 @@ public class Dynamic<T> extends DynamicLike<T> {
+         return new OptionalDynamic<>(ops, ops.getMap(value).flatMap(m -> {
+             final T value = m.get(key);
+             if (value == null) {
+-                return DataResult.error(() -> "key missing: " + key + " in " + this.value);
++                return DataResult.error(() -> DEBUG_MISSING_KEYS ? "key missing: " + key + " in " + this.value : "key missing: " + key); // Paper - Perf: Skip toString on values like NBT
+             }
+             return DataResult.success(new Dynamic<>(ops, value));
+         }));