Support for signing outgoing emails and optional encryption via WKD

[ZzenlD]

💡 The Idea
It would be great if emails sent by email could be signed with a private key so that the recipient can verify the authenticity of the email.
If the corresponding public key has been stored via WKD, this can be used by the recipient without manual configuration.
It would also be possible for WKD to automatically recognize the public key (if available) and additionally encrypt outgoing emails.

Both measures significantly increase security.

🔨 Breaking Feature
Both are merely optional extensions, therefore no breaking changes.

[caronc]

Request also came from #414 where PGP was added to the email.

I need to investigate how difficult it would be to also support WKD