Add support for custom headers

A complication is that there are various different types of header with different data/parameters. See Symfony\Component\Mime\Header\Headers.

Backported MR from https://www.drupal.org/project/symfony_mailer/issues/3511206#comment-161... ✨ How to add a custom header MessageStream: broadcast for Postmark? Active

Note that only text headers are supported. Maybe this Email Adjuster may be renamed "text header" instead of "custom header".

ressa → changed the visibility of the branch 2.x to hidden.

Thanks @opi, should status here be "Needs review" as well?

indeed