[Feature] Support configuration of SMTP username

[orilights]

1Panel Version

v2.0.11

Please describe your needs or suggestions for improvements

希望告警通知中，SMTP 用户名可单独配置，而不是使用发件邮箱

部分发件服务（如 Resend）需要使用固定用户名而不是邮箱地址来登录

https://resend.com/docs/send-with-smtp#smtp-credentials

Please describe the solution you suggest

No response

Additional Information

No response

[wanghe-fit2cloud]

感谢反馈，我们会在后续版本考虑支持这个需求。