Add Velocity IP forwarding support (#1557)

While Velocity supports BungeeCord-style IP forwarding, it is not secure. Users
have a lot of problems setting up firewalls or setting up plugins like IPWhitelist.
Further, the BungeeCord IP forwarding protocol still retains essentially its original
form, when there is brand new support for custom login plugin messages in 1.13.

Velocity's modern IP forwarding uses an HMAC-SHA256 code to ensure authenticity
of messages, is packed into a binary format that is smaller than BungeeCord's
forwarding, and is integrated into the Minecraft login process by using the 1.13
login plugin message packet.

Spigot-Server-Patches/Configurable-connection-throttle-kick-message.patch

@@ -5,7 +5,7 @@ Subject: [PATCH] Configurable connection throttle kick message
 
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperConfig.java b/src/main/java/com/destroystokyo/paper/PaperConfig.java
-index 454ac2c091..03c15e7c9f 100644
+index 77d35ac99..352166725 100644
 --- a/src/main/java/com/destroystokyo/paper/PaperConfig.java
 +++ b/src/main/java/com/destroystokyo/paper/PaperConfig.java
 @@ -0,0 +0,0 @@ public class PaperConfig {
@@ -21,7 +21,7 @@ index 454ac2c091..03c15e7c9f 100644
      private static void savePlayerData() {
          savePlayerData = getBoolean("settings.save-player-data", savePlayerData);
 diff --git a/src/main/java/net/minecraft/server/HandshakeListener.java b/src/main/java/net/minecraft/server/HandshakeListener.java
-index a02fa29217..7dc51fd995 100644
+index a02fa2921..7dc51fd99 100644
 --- a/src/main/java/net/minecraft/server/HandshakeListener.java
 +++ b/src/main/java/net/minecraft/server/HandshakeListener.java
 @@ -0,0 +0,0 @@ public class HandshakeListener implements PacketHandshakingInListener {