Add support for signing and encryption

Good idea. In addition to S/MIME signing could also include DKIM signing and encrypting.

Hi,

I attach a patch with following changes :

• Introduces a new EmailAdjuster plugin: DkimSignerAdjuster, with configurable domain / selector and keys generation / DNS information
• Add a pre-send phase (needed to allow changes directly on fully-prepared symfony email, as described by symfony mailer's DkimSigner documentation)
• Introduces a new SigningHelper service (to factor methods like keys discovery/generation)

Fix configuration variables + improve dkim keys informations

Could someone review this code?
It concerns only DKIM signing, but could be useful and merged IMHO (more and more email providers block unsigned emails).

After testing on another project, I add test for DKIM domain on plugin's settings form.

I found this issue when I proposed a similar feature for a friendly module in the neighborhood and I think it worth mentoining it seems that the Symfony Mailer library may have a built-in event to add this feature: https://www.drupal.org/project/symfony_mailer_lite/issues/3424075#commen... ✨ Add support form DKIM signing Active

I haven't checked yet what other possibilities are available in this module.

Thanks @mxr576 for this. Could you try this module with #7 patch to confirm that it works ?