JMAP support

[alexanderadam]

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

JMAP is like IMAP but in a modern way. There's a detailed description on jmap.io, though.
Clients like Ltt.rs, Group Office or meli are supporting it already.
And having JMAP in more email server solutions might increase its popularity.

Describe the solution you'd like

Maybe it would be nice to integrate a JMAP Proxy server in the Docker setup until Dovecot supports it natively.

Describe alternatives you've considered

I guess using IMAP?

PS: Cyrus IMAPD can also be compiled with JMAP support, but I don't think it would be useful to switch to Cyrus IMAPD