Post a manual channel commission

Manually post a channel commission line on a reservation folio. This covers the edge case where a reservation that should have been channel-delivered was created manually in the PMS, so no commission line was posted automatically. The commission is posted under the same transaction code as channel-delivered commission (so it rolls up identically in cost-of-acquisition reporting) but is flagged as manually sourced, which allows multiple manual commissions per reservation and makes the line voidable from the folio. Processing is asynchronous; the returned event id can be used to track completion.