Add support for Plugin Message packet, full implementation later.

src/main/java/com/velocitypowered/proxy/protocol/StateRegistry.java

@@ -35,6 +35,10 @@ public enum StateRegistry {
                     map(0x02, MINECRAFT_1_11),
                     map(0x03, MINECRAFT_1_12),
                     map(0x02, MINECRAFT_1_12_2));
+            SERVERBOUND.register(PluginMessage.class, PluginMessage::new,
+                    map(0x0A, MINECRAFT_1_11),
+                    map(0x0A, MINECRAFT_1_12),
+                    map(0x09, MINECRAFT_1_12_1));
             SERVERBOUND.register(KeepAlive.class, KeepAlive::new,
                     map(0x0B, MINECRAFT_1_11),
                     map(0x0C, MINECRAFT_1_12),
@@ -48,6 +52,8 @@ public enum StateRegistry {
                     map(0x0C, MINECRAFT_1_11));
             CLIENTBOUND.register(Chat.class, Chat::new,
                     map(0x0F, MINECRAFT_1_11));
+            CLIENTBOUND.register(PluginMessage.class, PluginMessage::new,
+                    map(0x18, MINECRAFT_1_11));
             CLIENTBOUND.register(Disconnect.class, Disconnect::new,
                     map(0x1A, MINECRAFT_1_11));
             CLIENTBOUND.register(KeepAlive.class, KeepAlive::new,

src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftVarintLengthEncoder.java

@@ -14,6 +14,7 @@ public class MinecraftVarintLengthEncoder extends MessageToByteEncoder<ByteBuf>
 
     @Override
     protected void encode(ChannelHandlerContext ctx, ByteBuf msg, ByteBuf out) throws Exception {
+        out.ensureWritable(msg.readableBytes() + 5);
         ProtocolUtils.writeVarInt(out, msg.readableBytes());
         out.writeBytes(msg);
     }

src/main/java/com/velocitypowered/proxy/protocol/packets/PluginMessage.java

@@ -0,0 +1,24 @@
+package com.velocitypowered.proxy.protocol.packets;
+
+import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.ProtocolConstants;
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import io.netty.buffer.ByteBuf;
+
+public class PluginMessage implements MinecraftPacket {
+    private String channel;
+    private byte[] data;
+
+    @Override
+    public void decode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        this.channel = ProtocolUtils.readString(buf, 20);
+        this.data = new byte[buf.readableBytes()];
+        buf.readBytes(this.data);
+    }
+
+    @Override
+    public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        ProtocolUtils.writeString(buf, channel);
+        buf.writeBytes(data);
+    }
+}