Implement SCHEDULE-FORCE-SEND support (RFC 6638 §3.2.7)

Is your feature request related to a problem? Please describe.

There is currently no way to resend a calendar invitation to an attendee who has not responded. The only workaround is to remove the attendee and re-add them, which sends an unwanted cancellation email first. This is tracked as a user-facing issue in nextcloud/calendar#4019 (https://github.com/nextcloud/calendar/issues/4019).

The root blocker is missing support for SCHEDULE-FORCE-SEND in calendar-js.

Describe the solution you'd like

Implement support for the SCHEDULE-FORCE-SEND iCalendar property parameter as defined in RFC 6638 §3.2.7 (https://datatracker.ietf.org/doc/html/rfc6638#section-3.2.7). This parameter instructs the CalDAV server to send a scheduling message to an attendee or organizer even in cases where it would not normally do so, enabling a "resend invitation" flow without modifying the event itself.

RFC 6638 is a finalized Proposed Standard published in June 2012, so the specification is stable and unambiguous.

Describe alternatives you've considered

The remove-and-re-add workaround (referenced in nextcloud/calendar#4019) is the only current option. It is disruptive as it sends a spurious cancellation email to the attendee before the new invitation arrives.

Additional context

This is a prerequisite for implementing the "resend invitation" button requested in nextcloud/calendar#4019 (https://github.com/nextcloud/calendar/issues/4019). Once SCHEDULE-FORCE-SEND is available in calendar-js, the calendar app can expose a per-attendee resend action in the UI.

Start by locating calendar-js's handling of iCalendar property parameters, then read RFC 6638 §3.2.7 alongside the existing scheduling implementation. Done means calendar-js supports SCHEDULE-FORCE-SEND so a CalDAV server can be instructed to resend a scheduling message without modifying the event.