Feat: Add SMTP DKIM settings

[chakflying]

Description

Closes #1030

This should just work as we just exposed the nodemailer's DKIM settings. I don't have a mailserver setup to test tho.

Type of change

• User Interface
• New feature (non-breaking change which adds functionality)
• Translation update

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and test it
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• My code needed automated testing. I have added them (this is optional task)

Screenshots (if any)

cc. @ilogus

[ilogus]

Hi @chakflying, I just tested your fork, everything is good on my side ! ✅

Here are some screens :

If you want to test by yourself, and modify the options, contact me by email I could send you smtp credentials and a private key.

Thank you for this feature, in the hope that it will be fully validated !

[andreasbrett]

Awesome! Can't wait to see this in production.

[yeya]

@ilogus You should not post your private key or any part of it in public

[ilogus]

@ilogus You should not post your private key or any part of it in public

Don't worry, this mailbox, the DNS record with the public key and the private key were only used for this test. In order to be able to take a picture of this for you