Prototype: setPaymentMethodOnCart

[pmclain]

Description (*)

Implements mutation for setting cart payment method. Additionally provides a convention for handling the dynamic additional_data object currently sent with REST payment information.

The input PaymentMethodInput provides payment integrators the ability to define additional inputs for populating additional_data. I've included a partial example using the Braintree integration https://github.com/magento/graphql-ce/compare/2.3-develop...pmclain:issue/293?expand=1#diff-9cca29ccf0ab73e1ac28e56811a208a8.

Custom input types can converted to additional_data by registering a Magento\QuoteGraphQl\Model\Cart\Payment\AdditionalDataBuilderInterface for the payment method with the Magento\QuoteGraphQl\Model\Cart\Payment\AdditionalDataBuilderPool. The default assumption is a one-to-one match of input property to the desired array set as the payment method additional data.

Payment methods like Check/MO that do not have a dependency on the additional_data property work as-is and do not require registering an AdditionalDataBuilderInterface

Fixed Issues (if relevant)

1. [Payment methods] Set Payment Method on Cart #293
   Add support for Payment Methods:
   [Payment methods] Support of "Check / Money Order" payment method #286 - Check/Money Order
   [Payment methods] Support of "Bank Transfer Payment" method #287 - Bank Transfer
   [Payment methods] Support of "Cash On Delivery Payment" method #288 - Cash on Delivery
   [Payment methods] Support of "Purchase Order" payment method #290 - Purchase Order
   Braintree and Braintree Vault (requires test coverage)

Manual testing scenarios (*)

1. Create customer token
2. Create empty cart for customer
3. Using the customer token and masked quote id set payment method on cart with the mutation below:

mutation {
  setPaymentMethodOnCart(input: 
    {
      cart_id: "{{maskedQuoteId}}", 
      payment_method: {
          method: "{{methodCode}}"
        }
      }) {
    
    cart {
      cart_id,
      payment_method {
        method
      }
    }
  }
}

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)

[paliarush]

It is better to use full field name "purchase_order_number". Is there a reason to shorten it here?
Same on line 206

[ghost]

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[keharper]

Documented as part of magento/devdocs#4502