invitationPreviewKey function
Where inviter publishes the preview of invitation id:
public:_<id>.invitations.<namespace><inviter>.
A public:_… record is never listed by a scan, so only someone holding
the invitation id can read it.
Implementation
@visibleForTesting
AtKey invitationPreviewKey(Atsign inviter, String id, String namespace) =>
AtKey.public('_$id', namespace: 'invitations.$namespace', sharedBy: inviter)
.build();