(tools, server) feat: Replace TCP server with gRPC server

Tools/ProtoBuf/proto/message.proto

@@ -1,3 +1,52 @@
 syntax = "proto3";
 
-option csharp_namespace = "Protocol";
+package protocol;
+
+// Define services
+
+service GeneralService {
+    // Get server info from server.
+    //
+    // This parameter actually doesn't accept any arguments,
+    // but it is still required owing to Protobuf grammar.
+    rpc GetServerInfo(Empty) returns (ServerInfo);
+}
+
+service GameService {
+    rpc Login(LoginRequest) returns (LoginResponse);
+    rpc Signup(SignupRequest) returns (SignupResponse);
+}
+
+// Define messages
+
+enum RequestResult {
+    Success = 0;
+    Fail = 1;
+}
+
+message Empty {}
+
+message ServerInfo {
+    string Lang = 1;
+    string ver = 2;
+}
+
+message LoginRequest {
+    string Username = 1;
+    string Password = 2;
+}
+
+message LoginResponse {
+    RequestResult result = 1;
+    string message = 2;
+}
+
+message SignupRequest {
+    string Username = 1;
+    string Password = 2;
+}
+
+message SignupResponse {
+    RequestResult result = 1;
+    string message = 2;
+}