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/Allow-specifying-a-custom-authentication-servers-dow.patch

@@ -6,7 +6,7 @@ Subject: [PATCH] Allow specifying a custom "authentication servers down" kick
 
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperConfig.java b/src/main/java/com/destroystokyo/paper/PaperConfig.java
-index 56cd386f34..2d98052ad1 100644
+index 963096fb3..a499578db 100644
 --- a/src/main/java/com/destroystokyo/paper/PaperConfig.java
 +++ b/src/main/java/com/destroystokyo/paper/PaperConfig.java
 @@ -0,0 +0,0 @@
@@ -27,7 +27,7 @@ index 56cd386f34..2d98052ad1 100644
 +    }
  }
 diff --git a/src/main/java/net/minecraft/server/LoginListener.java b/src/main/java/net/minecraft/server/LoginListener.java
-index 8bbea96240..10c82107d4 100644
+index 8bbea9624..10c82107d 100644
 --- a/src/main/java/net/minecraft/server/LoginListener.java
 +++ b/src/main/java/net/minecraft/server/LoginListener.java
 @@ -0,0 +0,0 @@ public class LoginListener implements PacketLoginInListener, ITickable {