:skip-function in org-caldav-calendars sets org-agenda-skip-function

Usage:

    (setq org-caldav-calendars
     ((:calendar-id "org-home"
       :skip-function org-agenda-skip-work
       :inbox "~/org/caldav-inbox-home.org")
      (:calendar-id "org-work"
       :skip-function (org-agenda-skip-work 'inverse)
       :inbox "~/org/caldav-inbox-work.org")))

Could probably be an org-caldav variable that is then set first thing in
`org-caldav-generate-ics' using an outer

    (let ((org-agenda-skip-function org-caldav-skip-function))
        rest-of-function)

if that feels nicer …