Add adapter for JMAP APIs

[jaudriga]

The JSON Meta Application Protocol (JMAP) is an upcoming alternative (or even successor) to IMAP/CalDAV/CardDAV. Similar to #333 we would like to add support for JMAP to DTP.

Ideally this should happen via a generic JMAP adapter for which an endpoint can be chosen by the user (see #943) and (since most current providers providing JMAP APIs do not support OAuth) this should ideally be compatible with other auth mechanisms as well (see #944).