Include login_name in adminCreateUser response (#20283)

`login_name` (Authentication Sign-in Name) is not included in the response of `adminUserCreate` API. 
This PR is to return user-specified `login_name` if there is one.

modules/structs/user.go

@@ -17,6 +17,9 @@ type User struct {
 	ID int64 `json:"id"`
 	// the user's username
 	UserName string `json:"login"`
+	// the user's authentication sign-in name.
+	// default: empty
+	LoginName string `json:"login_name"`
 	// the user's full name
 	FullName string `json:"full_name"`
 	// swagger:strfmt email