Provide available shipping rates for addresses

[pmclain]

Description (*)

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

This continues building some prerequisites for #39 #285

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
        }
      }
    }
  }
}

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[naydav]

Instead of using second parameter-flag we can move this logic to separate class

[naydav]

If we declare separate resolver for available_shipping_methods in a scheme we don't need this class

[naydav]

We can split logic between different resolvers
Like addresses: [CartAddress]! @resolver(class: "\\Magento\\QuoteGraphQl\\Model\\Resolver\\CartAddresses")
It will simplify our code and relations

[pmclain]

@naydav Thanks for checking this out. I've separated the resolver for the shipping methods 2f7237a

[ghost]

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[keharper]

Documented as part of magento/devdocs#4502