Empty cart creation added

[rogyar]

Description

Implementation of mutation for empty cart creation by the guest/customer user.
createEmptyCart mutation is declared which accepts no arguments. It returns a hashed cart ID for guest customers and the real hashed quote ID for logged in customers. The customer status (logged in/guest) is performed by checking authorization headers of the request.

Fixed Issues (if relevant)

1. [Cart Operations] Create Empty Cart #118 : [Mutations] Cart Operations > Create Empty Cart

Manual testing scenarios

Use the following query to create an empty shopping cart:

mutation {
  createEmptyCart
}

To test with logged in customer, obtain the customer token and provide it in the request headers.

[naydav]

Looks like we need to add new module to
composer.json (and regenerate composer.lock)

[paliarush]

@rogyar I have modified requirements slightly. We need to return cart ID hash for registered customers as well. Please let me know if you could do that change in current task, otherwise we will create a separate one.

Sorry for the change, but this decision was made after the task was created.

composer.json and composer.json added. Review is dismissed to unblock merge

[paliarush]

We should avoid working with global $context object, it is possible to inject UserContextInterface via constructor instead.

[rogyar]

That was my previous approach. But I saw that we already have UserContextInterface within a scope of $context and it doesn't make sense to inject it once again.
Thank you for the tip, I will roll back the changes