Support Symfony Mailer

Could one of the people who worked on this please respond to jonathanjfshaw's comments on the merge request? Thank you.

I think plugin should be automatic by automatic = TRUE,:

 * @EmailAdjuster(
 *   id = "maillog",
 *   label = @Translation("Maillog"),
 *   description = @Translation("Use Maillog handling."),
 *   weight = 100000,
 *   automatic = TRUE,
 * )

Currently, to make it work, you have to add it manually, and this is stored in configuration, means, that you have to bring maillog to production because of that plugin. By making it automatic, Symfony Mailer will include it for every email and won't affect any configuration changes. Anyway this is controller by Maillog settings, not by adjuster.

We need to add this to the next release.

Having thought through it a bit for a while, I think moving entirely to Symfony Mailer → and then using Symfony Mailer Log → is the best long term plan for the Drupal community. As such, I don't see any reason to add Symfony Mailer support to Maillog.