Find an existing shipment by shipment id.
Returns the V2 payload (PublicAPIShipmentDetailsV2) when the "Public API & Webhook V2 Payload Parity"
feature is enabled for the site; the V2 payload renders referenceType as a free-form string and includes
customer-configured custom reference numbers at the shipment, stop, and commodity levels. Otherwise returns
the legacy V1 payload (PublicAPIShipmentDetails) with referenceType constrained to the ShipmentReferenceType enum.