Add AddGenHeaderPreformatted

[thib-d]

Is your feature request related to a problem? Please describe.

In teh lib dkim could be nice to have 2 dkim key (So 2 middlewahre).
But the lib https://github.com/wneessen/go-mail-middleware/blob/main/dkim/dkim.go#L94 do SetGenHeaderPreformatted and not AddGenHeaderPreformatted. So it not possble.

Describe the solution you'd like

What need to be done :
Create AddGenHeaderPreformatted function, and edit go-mail-middleware for it

Describe alternatives you've considered

No response

Additional context

No response

[wneessen]

Hi @thib-d,

thanks for the suggestion. That might be an interesting enhancement indeed. Unfortunately I currently don't have the time to work on this. But if you feel comfortable to do it yourself, please feel free to submit a PR.

Since I am working on a DKIM integration directly into go-mail on the side, I'll keep this in mind for that as well.