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/Make-player-data-saving-configurable.patch

@@ -5,7 +5,7 @@ Subject: [PATCH] Make player data saving configurable
 
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperConfig.java b/src/main/java/com/destroystokyo/paper/PaperConfig.java
-index 2d98052ad1..588c626fbd 100644
+index a499578db..dc15bfcf8 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 {
@@ -23,7 +23,7 @@ index 2d98052ad1..588c626fbd 100644
 +    }
  }
 diff --git a/src/main/java/net/minecraft/server/WorldNBTStorage.java b/src/main/java/net/minecraft/server/WorldNBTStorage.java
-index 0fd6efec0b..7553280d21 100644
+index 0fd6efec0..7553280d2 100644
 --- a/src/main/java/net/minecraft/server/WorldNBTStorage.java
 +++ b/src/main/java/net/minecraft/server/WorldNBTStorage.java
 @@ -0,0 +0,0 @@ public class WorldNBTStorage implements IDataManager, IPlayerFileData {