feat: add support for smtp when send email

[jonahcui]

虽然 resend 很好用，但是 smtp 还是要支持一下的，支持使用 smtp 发送邀请邮件

[crazywoola]

Please add my wechat: crazyphage, I will invite to you our developers group.

[takatost]

So here we're using app.config.get('xxx') to get configuration variables, but it seems like there's no logic declared in config.py to fetch the environment variables?

[jonahcui]

So here we're using app.config.get('xxx') to get configuration variables, but it seems like there's no logic declared in config.py to fetch the environment variables?

Thank u, I changed the config.py and submitted it.

[takatost]

Thank you so much! 😊
BTW, would it be possible for you to help us add some explanations about these environment variables in the documentation (both in English and Chinese)? We would really appreciate it. 🙏
https://github.com/langgenius/dify-docs/blob/main/en/getting-started/install-self-hosted/environments.md#mail-related-configuration
https://github.com/langgenius/dify-docs/blob/main/zh_CN/getting-started/install-self-hosted/environments.md#%E9%82%AE%E4%BB%B6%E7%9B%B8%E5%85%B3%E9%85%8D%E7%BD%AE