Allow comments to organizer in icalendar event replies (Bug#72831)

Allow comments to organizer in icalendar event replies (Bug#72831)

* lisp/gnus/gnus-icalendar.el
(gnus-icalendar-event--build-reply-event-body): Add optional COMMENT
argument to be inserted into the reply.
(gnus-icalendar-event-reply-from-buffer): Add COMMENT argument to be
passed through to gnus-icalendar-event--build-reply-event-body
(gnus-icalendar-reply-accept, gnus-icalendar-reply-tentative,
gnus-icalendar-reply-decline): If interactively called with a prefix
argument ask user for a COMMENT to add to the reply.

* test/lisp/gnus/gnus-icalendar-tests.el
(gnus-icalendar-accept-with-comment,
gnus-icalendar-decline-without-changing-comment): New tests.

(cherry picked from commit 8332b4dd07a43d09ff8eed7097873d9ac4d5afc8)