[Checkout coverage] Place order

[misha-kotov]

As a Magento developer, I need to create and modify orders via GraphQL so that I can build basic ecommerce experiences for shoppers on the front-end using only GraphQL.

GraphQL needs to provide sufficient mutations for a developer to enable order placement for shoppers. Creating and modifying the Cart object will be done in Cart Operations story

Use cases:

• Guest and registered shoppers can place orders during checkout
• Registered shoppers can "reorder" from their past orders in My Account

Acceptance criteria:

• GraphQL provides a mutation through which registered and guest shoppers can place orders
• Editing of orders is out of scope since customers cannot take actions on placed orders
• Changes are covered with web API functional tests. See \Magento\GraphQl\Catalog\ProductViewTest::testQueryAllFieldsSimpleProduct as an example.

Storefront areas impacted:

• Checkout