implement McoinExchange packet Handler

src/main/java/emu/grasscutter/server/packet/recv/HandlerMcoinExchangeHcoinReq.java

+package emu.grasscutter.server.packet.recv;
+
+import emu.grasscutter.net.packet.Opcodes;
+import emu.grasscutter.net.packet.PacketHandler;
+import emu.grasscutter.net.packet.PacketOpcodes;
+import emu.grasscutter.net.proto.McoinExchangeHcoinReqOuterClass;
+import emu.grasscutter.server.game.GameSession;
+import emu.grasscutter.server.packet.send.PacketMcoinExchangeHcoinRsp;
+
+@Opcodes(PacketOpcodes.McoinExchangeHcoinReq)
+public class HandlerMcoinExchangeHcoinReq extends PacketHandler {
+
+    @Override
+    public void handle(GameSession session, byte[] header, byte[] payload) throws Exception {
+        McoinExchangeHcoinReqOuterClass.McoinExchangeHcoinReq exchangeReq = McoinExchangeHcoinReqOuterClass.McoinExchangeHcoinReq.parseFrom(payload);
+        if (session == null) {
+            return;
+        } else if (session.getPlayer().getCrystals() < exchangeReq.getMCoinNum()) {
+            return;
+        }
+        session.getPlayer().setCrystals(session.getPlayer().getCrystals() - exchangeReq.getMCoinNum());
+        session.getPlayer().setPrimogems(session.getPlayer().getPrimogems() + exchangeReq.getHCoinNum());
+        session.getPlayer().save();
+        session.send(new PacketMcoinExchangeHcoinRsp(session.getPlayer().getCrystals(), session.getPlayer().getPrimogems()));
+    }
+}

src/main/java/emu/grasscutter/server/packet/send/PacketMcoinExchangeHcoinRsp.java

+package emu.grasscutter.server.packet.send;
+
+import emu.grasscutter.net.packet.BasePacket;
+import emu.grasscutter.net.packet.PacketOpcodes;
+import emu.grasscutter.net.proto.McoinExchangeHcoinRspOuterClass;
+
+public class PacketMcoinExchangeHcoinRsp extends BasePacket {
+
+    public PacketMcoinExchangeHcoinRsp(int mcoin, int hcoin) {
+        super(PacketOpcodes.McoinExchangeHcoinRsp);
+
+        McoinExchangeHcoinRspOuterClass.McoinExchangeHcoinRsp mcoinExchangeHcoinRsp = McoinExchangeHcoinRspOuterClass.McoinExchangeHcoinRsp.newBuilder()
+                .setMCoinNum(mcoin)
+                .setHCoinNum(hcoin).build();
+
+        this.setData(mcoinExchangeHcoinRsp);
+    }
+}