Shipping ownership mode

Vendors always dispatch their own part of an order (one shipment per vendor). What is configurable is shipping.method_owner: vendor (the vendor creates their own shipping methods and manages the shipments) or operator (the operator defines the shipping methods; the buyer is charged once even though the order still splits into one shipment per vendor). vendor is the default.

# config/packages/odiseo_sylius_marketplace.yaml
odiseo_sylius_marketplace:
    shipping:
        method_owner: vendor