Add Merge Cart Mutation

[pmclain]

Description (*)

Add mutation for merging carts.

Guest Use Cases:

• Merge two guest carts

Customer Use Cases:

• Merge customer cart with guest cart
• Merge customer cart with inactive customer cart

Fixed Issues

1. [Checkout] Implement mergeCarts #905

Manual testing scenarios (*)

1. Create two carts
2. Add products to each cart

mutation {
  mergeCarts(input: {
    first_cart_id: "{$firstMaskedId}"
    second_cart_id: "{$secondMaskedId}"
  })
}

Expected Result
Cart contents are merged. A new masked cart id is returned and the previous masked ids return a not found error.

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 are green)

[lenaorobei]

Need to move this PR to on hold until we will finalize product requirements with @nrkapoor
magento/architecture#288

[lenaorobei]

Sorry to say that but I need to close this PR due to requirement changes. The new flow we'll be working on:

[ghost]

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[ghost]

Hi @pmclain, thank you for your contribution!
Please, complete Contribution Survey, it will take less than a minute.
Your feedback will help us to improve contribution process.