DAV resource: support exception incidences

My calendar has a few events like
[...]
BEGIN:VEVENT   # the only one
DTSTAMP:20210510T164410Z
CREATED:20210510T164409Z
LAST-MODIFIED:20210510T164410Z
DTSTART;TZID=Europe/Paris:20210511T143000
DTEND;TZID=Europe/Paris:20210511T153000
RECURRENCE-ID;TZID=Europe/Paris:20210513T143000
[...]

They show up in roundcube but were missing in korganizer.
I'm no ICAL expert but I think those are exceptions
to a recurring event, detached from that recurring event.

And indeed after this fix, they show up as "Recurrence: Exception"
in korganizer.

Found by debugging "Failed to parse item data" warnings from
the DAV resource.
1 job for release/21.08 in 10 minutes and 20 seconds (queued for 25 minutes and 28 seconds)
Status Name Job ID Coverage
  Build
failed build_clazy_clang_tidy #128240

00:10:20

 
Name Stage Failure
failed
build_clazy_clang_tidy Build
[ 93%] Linking CXX executable ../../bin/akonadi_ews_resource
[ 93%] Built target akonadi_imap_resource
[ 93%] Built target akonadi_ews_resource
make: *** [Makefile:146: all] Error 2
Uploading artifacts for failed job
Uploading artifacts...
WARNING: JUnitTestResults.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1