Add external transactional endpoint (#1108)

[Facecube]

Hello @knadh,
I am tagging you because you were also involved in the linked issue.

I decided to create a proposal implementation that allows external mails to be sent.
I implemented it as a separate endpoint in order not to interfere with the existing transactional endpoint.
I tested it locally and it seems to work for me.

Prior to this PR I have not written a single line of go, so please go easy on me :).
Looking forward to your feedback and I am happy to make changes, as I really would like to see this feature added to listmonk.

[knadh]

Hi @Facecube. I missed reviewing this last month in the middle of the v3.0.0 release I think. Will look into this sometime in the coming weeks.

[ugd]

@knadh, I forked the @Facecube solution, merged with the current release, set it up in a Docker container, and successfully deployed it in production; it has already sent around 60 emails. A useful enhancement would be to add a page for viewing the logs of sent emails.

[Facecube]

@ugd there is a logs page in the settings submenu.
But I assume you mean that you want some sort of list view of emails sent, where you can maybe even see the message body, etc.
But cool to see that my changes are actually usable 😆