Add a MinecraftVarintLengthCompositeEncoder for backend connections

This is an optimization that has been in BungeeCord for some time, but it's an idea that makes a lot of sense: avoid a memory copy when we don't need to do one. The OS will hopefully be smart enough to use `writev()` or similar for this.

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

@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2018-2023 Velocity Contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package com.velocitypowered.proxy.protocol.netty;
+
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import io.netty.buffer.ByteBuf;
+import io.netty.channel.ChannelHandler;
+import io.netty.channel.ChannelHandlerContext;
+import io.netty.handler.codec.MessageToMessageEncoder;
+import java.util.List;
+
+/**
+ * Handler for appending a length for Minecraft packets using composite buffers.
+ */
+@ChannelHandler.Sharable
+public class MinecraftVarintLengthCompositeEncoder extends MessageToMessageEncoder<ByteBuf> {
+
+  public static final MinecraftVarintLengthCompositeEncoder INSTANCE = new MinecraftVarintLengthCompositeEncoder();
+
+  private MinecraftVarintLengthCompositeEncoder() {
+  }
+
+  @Override
+  protected void encode(ChannelHandlerContext ctx, ByteBuf buf,
+      List<Object> list) throws Exception {
+    ByteBuf varIntBuffer = ctx.alloc().ioBuffer(ProtocolUtils.varIntBytes(buf.readableBytes()));
+    ProtocolUtils.writeVarInt(varIntBuffer, buf.readableBytes());
+    list.add(varIntBuffer);
+    list.add(buf.retain());
+  }
+}