Resend NGC invitations to pending attendees of a training event.

Ask NGC to resend the external user group invitation email to one or more attendees whose invitation is still pending (e.g. missed or lost the original email). Attendees who have already confirmed their invitation are rejected. Does not modify the training's attendees list or any existing workbench simulations. Use the `external-user-group` action to see which attendees are still pending before calling this. **Required roles (all):** `air_instructor`, `air_org_admin`, `user_admin` **Additional permissions:** - Only the instructor who created this training may perform this action. - The client must have the NCA 'Administrator' role set in the NCA account associated with the NGC Organization.