Define a single exchange rate per shipment, applied to all its invoices

Allow a single exchange rate to be defined at the shipment level and applied consistently to every invoice generated for that shipment, so currency conversion stays uniform across all of the shipment's billing. Today the rate is set automatically from the transaction date, which can produce different rates across invoices within the same shipment and complicate profit/loss control. The shipment-level rate should be editable and settable through the API.