Setting shipping method for shopping cart (single address shipping)

[rogyar]

Description (*)

This PR introduces mutation for setting shipping method for a shopping cart. Also, this PR contains GraphQL schema for working with different checkout operations (setting billing and shipping addresses etc.)

Fixed Issues (if relevant)

#386

Manual testing scenarios (*)

1. Create an empty shopping cart and add some products to the shopping cart
2. Use the following query (with the corresponding shopping cart id) to set a shipping method:

mutation {
  setShippingMethodsOnCart(input: 
    {
      cart_id: "fda39cfe15aabde6d3c362cb3878de5s", 
      shipping_methods: [
        {
          shipping_method_code: "bestway"
          shipping_carrier_code: "tablerate"
          cart_address_id: 275
        }
      ]}) {
    
    cart {
      cart_id,
      addresses {
        firstname
        lastname
        company
        address_type
        city
        street
        region {
          code
          label
        }
        postcode
        country {
          code
          label
        }
        
        selected_shipping_method {
          code
          label
        }
      }
    }
  }
}

[keharper]

Documentation completed in magento/devdocs#4502