Extract JMAP module from python-caldav into standalone package

[SashankBhamidi]

The JMAP code lives in caldav/jmap/ inside python-caldav. This is where it lives from now on.

Everything under caldav/jmap/ moves here, including the tests. The main coupling to cut is HTTPBearerAuth from caldav/requests.py. Either move it or reimplement it. Check for any other caldav.* imports in the jmap module and cut each one.

The config integration (get_jmap_client reading from caldav config files) stays in python-caldav as a compatibility shim.

• Copy files, fix imports (caldav.jmap.* -> calendaring_jmap.*)
• Cut all caldav.* dependencies
• test_jmap_unit.py passes with no python-caldav installed
• Open PR in python-caldav to add optional dep and re-export
• Add deprecation notice in caldav.jmap pointing here

Reference: python-caldav/caldav#625.