Add support for total cost of cart to cart transform, including taxes and shipping

[kylebuildsstuff]

We're currently using cart transform functions to change the price of placeholder line items to add dynamic fees to the cart (e.g. package protection 5% of total cart cost).

query RunInput {
  cart {
    lines {
      id
      quantity
      cost {
        totalAmount {
          amount
          currencyCode
        }
      }
      ...

However this doesn't take into account taxes and shipping fees because the cart transform function doesn't have access to those inputs. For some of our Shopify Plus clients this means they have to eat the costs themselves. It would be great to have access to these inputs to account for the total cost of the cart, including taxes and shipping fees, or if there's another way to do this that would be great too.