Caldav: Google deprecated commonly used endpoint

[thucke]

The current examples of the Caldav Command Binding documentation for connection Google calendars are shoing URL examples like https://www.google.com/calendar/dav/[email protected]/events.
However the CalDAV API Developer's Guide states that this endpoint is deprecated and no longer supported.

Hence the binding should be extended to support Googles OAuth 2.0 authentication.
There were already activities and also a functional solution (see querdenker2k#2) that I personally used in OH 1.8.