[Shipping methods] Get list of available shipping methods for current cart

[naydav]

Description (*)

Provide ability to retrieve available shipping rates for all cart shipping addresses.
Available methods are quite different objects from selected methods, given the selected methods are really just attributes of quote addresses.

Manual testing scenarios (*)

1. Create empty cart and add shippable products
2. Set shipping address and request available_shipping_methods

mutation {
  setShippingAddressesOnCart(
    input: {
      cart_id: "$maskedQuoteId"
      shipping_addresses: [
        {
          address: {
            firstname: "test firstname"
            lastname: "test lastname"
            company: "test company"
            street: ["test street 1", "test street 2"]
            city: "test city"
            region: "test region"
            postcode: "887766"
            country_code: "US"
            telephone: "88776655"
            save_in_address_book: false
          }
        }
      ]
    }
  ) {
    cart {
      addresses {
        firstname
        lastname
        company
        street
        city
        postcode
        telephone
        available_shipping_methods {
          amount
          available
          base_amount
          carrier_code
          carrier_title
          error_message
          method_code
          method_title
          price_excl_tax
          price_incl_tax
        }
      }
    }
  }
}

[naydav]

#339