[enhancement]: WKD integration for PGP key autodiscovery & interoperability

[gomarcd]

Great work on this project! I stumbled across it shortly after losing motivation in my own effort to cobble something together, and it looks like this has most of what I had in mind and likely much better than I could've done, so thank you for that. Just testing it out now, and I got a basic setup behind Caddy going with relatively little friction.

One addition that would be very nice is WKD integration, for a better user experience in terms of interoperability with other WKD-enabled providers like Proton Mail. The desired outcome with that would be when somebody using a WKD enabled provider like Proton types in your email address as a recipient, it will automatically fetch your published PGP key and use it.

Some reading on WKD:

https://wiki.gnupg.org/WKDHosting

https://wiki.gnupg.org/WKD?ref=uriports.com#Implementations

https://florian.latzel.io/gnupg-micro-howto.html#web-key-directory-wkd

https://www.kuketz-blog.de/gnupg-web-key-directory-wkd-einrichten/

https://metacode.biz/openpgp/web-key-directory

In my abandoned personal project I added this alongside a script to automatically PGP encrypt outbound mail if a key is discovered (used this for inspiration: https://github.com/wjgauthier/postfix-wkd) and it seemed to work. Not too sure about the viability or how easily something like this may be integrated into Stalwart but I felt it might be worth throwing it out there. What do people think?

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