Make it configurable, which headers are signed by DKIM Merged

when signing mail with a DKIM Key, the following headers are being signed:

h=From:Subject:To:Message-ID:Date:MIME-Version:Content-type;

This should be made configurable, since we would like to create special headers AND let them sign by DKIM.

e.g.

One of our customers wants to send newsletter mails with "one-click-unsubscribe" headers.

In https://datatracker.ietf.org/doc/html/rfc8058#section-4 under requirements it says, that for those headers to work, they need to be signed. Currently this is not possible, which makes it necessary, to implement some kind of gateway for a small shop.

tldr: please make the DKIM h-Tag configurable.