RE NXT check-in attendance

For GiveCampus integrations and API.

When the RE NXT event export integration is enabled and healthy, event participants include RSVP status and an attended value from GC check-in. The integration can update the matched NXT participant's RSVP and attended values. This is not a promise of instant synchronization or successful matching for every attendee. Review event RENXT Errors and the appropriate constituent identity if a registration fails to export; resolving an ID mismatch is distinct from recording attendance in GC.