Restructure Order shipping data model

[michaelbromley]

Is your feature request related to a problem? Please describe.
Currently we represent the Shipping method & costs of an order as 3 properties on the Order entity:

shippingMethod: ShippingMethod
shipping: number;
shippingWithTax: number;

There are a couple of issues with this:

1. It makes it impossible to use more than 1 ShippingMethod per Order. Though uncommon, this may be a requirement, e.g. to enable certain items to be sent via one method, and a single item to be drop-shipped. I expect such things to mainly be handled later at the Fulfillment stage, but I can also foresee that it might also be required at the ShippingMethod stage e.g. in order to separate the charges more distinctly.

2. Shipping promotions (e.g. free shipping) are hacky right now - you need to just do a OrderPromotionAction and subtract the
   shipping amount from the Order. Tax-wise this is not really correct, since you would then be reducing the (prorated) price of every OrderItem, even though it is actually only the shipping being discounted.

Describe the solution you'd like

Create a new ShippingLine entity which looks like this:

@Entity()
export class ShippingLine extends VendureEntity {

    shippingMethodId: ID | null;

    shippingMethod: ShippingMethod | null;

    order: Order;

    price: number;

    priceWithTax: number;

    adjustments: Adjustment[];
}

and then an Order entity has reference to one or more of these:

export class Order {

  shippingLines: ShippingLine[];
}

Then we need to introduce a new type of PromotionAction: PromotionShippingAction which allows us to add discounts to the ShippingLine.adjustments array.

Describe alternatives you've considered
Just keeping it as-is. Since we're already doing pretty big breaking changes to the Order model in v0.18.0, we might as well get it all right now and have it done with.