Reapply "feat: 实现身份资料、对话密码、在线目录与群管理"

This reverts commit 16ece09a97.

internal/app/identity/errors.go

@@ -0,0 +1,7 @@
+package identity
+
+import "git.asio.asia/nixevol/NixMsg/internal/protocol"
+
+func errCode(code, msg string) *protocol.Error {
+	return &protocol.Error{Code: code, Message: msg}
+}