Added methods for working with external courier services

From V7 in delivery IDeliveryOrder a new field appears IDeliveryOrder.ExternalCourierServiceData, as well as methods for assigning an external courier service PluginContext.Operations.ChangeDeliveryExternalCourierService and external courier PluginContext.Operations.ChangeDeliveryExternalCourier.

Usage example

private void EcsExample()
{
    var delivery = ...;
    var credentials = PluginContext.Operations.AuthenticateByPin(pin);
    
    // ID received from external system
    Guid ecsId = ...;
    // System name
    string ecsName = "Service Name";

    PluginContext.Operations.ChangeDeliveryExternalCourierService(ecsId, ecsName, delivery, credentials);
    delivery = PluginContext.Operations.GetDeliveryOrderById(delivery.Id);

    string courierName = "John Johnson";
    string courierPhone = "+380971111011"; // courier phone number, in international format
    string courierComment = "Cash payment";

    PluginContext.Operations.ChangeDeliveryExternalCourier(courierName, courierPhone, courierComment, delivery, credentials)
}