[Checkout coverage] setGuestEmailOnCart mutation

[rogyar]

Description (*)

This PR introduces a possibility to set email for a guest quote. This step is necessary in case if an order is being placed for a guest.

Manual testing scenarios (*)

1. Create an empty cart as a guest user
2. Assign email to the quote in the following way

mutation {
  setGuestEmailOnCart(input: {
    cart_id:"$maskedCartId"
    email: ""
  }) {
    cart {
      guest_email
    }
  }
}

[ghost]

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[atwixfirster]

Assign email to the quote in the following way

mutation {
  setGuestEmailOnCart(input: {
    cart_id: "$maskedQuoteId"
    email: "$email"
  }) {
    cart {
      email
    }
  }
}

[keharper]

Added documentation in magento/devdocs#4502