Add items to cart in buk via Shop API mutation

[martijnvdbrug]

Is your feature request related to a problem? Please describe.

We would like to be able to add items to order in bulk, so that we can send links to customers that add a set of products to cart. E.g. shop.com/cart/add=1,2,3 will add variants 1, 2 and 3 to cart.

Describe the solution you'd like

The internal logic is already available in OrderService.addItemsToOrder(), it just needs to be exposed in the Shop API

Describe alternatives you've considered

Implement it in in our project, but it feels this should be available in core.

Additional context

I am willing to implement this, if it is agreed that it belongs in @vendure/core.