Add support for JMAP in addition to CalDAV

[dumblob]

JMAP is a quickly spreading standard for clients to connect to calendar, address, mail, etc. stores. It therefore intends to primarily accompany or replace CalDAV, CardDAV, IMAP, and SMTP. It is also designed to be more generic such that it can be extended in the future (this allows replacing CardDAV/CalDAV etc.). Note JMAP does not replace MTA-to-MTA SMTP transmission.

https://jmap.io/spec-calendars.html

About the JMAP working group at the IETF: https://datatracker.ietf.org/wg/jmap/about/

Thank you!

[dumblob]

@marclaporte might be of interest for you 😉.

[beedell-roke]

ietf-tools/datatracker/issues/6979#issuecomment-4415932647 may be of interest to you both.