Initial work on mail

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

+package emu.grasscutter.server.packet.recv;
+
+import emu.grasscutter.Grasscutter;
+import emu.grasscutter.net.packet.Opcodes;
+import emu.grasscutter.net.packet.PacketOpcodes;
+import emu.grasscutter.net.packet.PacketHandler;
+import emu.grasscutter.net.proto.GetAllMailReqOuterClass;
+import emu.grasscutter.net.proto.GetPlayerTokenReqOuterClass;
+import emu.grasscutter.server.game.GameSession;
+import emu.grasscutter.server.packet.send.PacketGetAllMailRsp;
+import emu.grasscutter.server.packet.send.PacketGetGachaInfoRsp;
+
+@Opcodes(PacketOpcodes.GetAllMailReq)
+public class HandlerGetAllMailReq extends PacketHandler {
+
+    @Override
+    public void handle(GameSession session, byte[] header, byte[] payload) throws Exception {
+        Grasscutter.getLogger().info("Mail Req");
+        GetAllMailReqOuterClass.GetAllMailReq req = GetAllMailReqOuterClass.GetAllMailReq.parseFrom(payload);
+        session.send(new PacketGetAllMailRsp(session.getPlayer(), req.getIsGiftMail()));
+    }
+
+}

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

+package emu.grasscutter.server.packet.send;
+
+import emu.grasscutter.game.GenshinPlayer;
+import emu.grasscutter.net.packet.GenshinPacket;
+import emu.grasscutter.net.packet.PacketOpcodes;
+import emu.grasscutter.net.proto.GetAllMailRspOuterClass.GetAllMailRsp;
+import emu.grasscutter.net.proto.MailDataOuterClass.MailData;
+import emu.grasscutter.net.proto.MailTextContentOuterClass.MailTextContent;
+
+import java.time.Instant;
+
+public class PacketGetAllMailRsp extends GenshinPacket {
+
+    public PacketGetAllMailRsp(GenshinPlayer player, boolean isGiftMail) {
+        super(PacketOpcodes.GetAllMailRsp);
+
+        GetAllMailRsp.Builder proto = GetAllMailRsp.newBuilder();
+
+        MailTextContent.Builder mailTextContent = MailTextContent.newBuilder();
+        mailTextContent.setTitle("System Message");
+        mailTextContent.setContent("I'm going to kill you...");
+        mailTextContent.setSender("YOU");
+
+        MailData.Builder mailData = MailData.newBuilder();
+        mailData.setMailId(0);
+        mailData.setMailTextContent(mailTextContent.build());
+        mailData.setSendTime((int) Instant.now().getEpochSecond());
+        mailData.setExpireTime(999999999);
+        mailData.setImportance(1);
+        mailData.setIsRead(false);
+        mailData.setIsAttachmentGot(false);
+        maildata.
+
+        proto.addMailList(mailData.build());
+        proto.setIsTruncated(false);
+
+        this.setData(proto);
+    }
+}