Checkout coverage

[paliarush]

As a Magento developer, I need to perform checkout operations via GraphQL so that I can build basic ecommerce experiences for shoppers on the front-end.

High Level use cases

• Cart manipulations
  -- Fetch user's cart
  -- Manage user's cart (addresses, items, qty, shipping, billing, payment details)
  -- Convert cart to order
• Set Payment method
• Set Shipping method

High Level approach to SDL

1. First build an API on top of current stack
2. Iterate to query on arbitrary attributes
3. Iterate to make it performant
4. Write tests that cover the desired end state
5. Write performance tests
6. Write unit tests
7. Write end-to-end tests