[Checkout Coverage] Place order for guest

[rogyar]

Description (*)

This PR adjusts place order resolver so guest is able to place order if guest email was previously set (#564). The quote details (customer firstname, lastname etc) are being populated from billing address as it's being performed for storefront checkout flow.

Manual testing scenarios (*)

1. Create empty cart
2. Add products to the shopping cart
3. Set billing address for the shopping cart
4. Set shipping address for the shopping cart
5. Set shipping method for the shopping cart
6. Set payment method for the shopping cart
7. Set guest email for the shopping cart ([Checkout coverage] setGuestEmailOnCart mutation #564). As a temporary solution while the setting guest email mutation is not merged, you can set an email for the corresponding quote manually in the database in quote table.
8. Place order using the following mutation:

mutation {
  placeOrder(input: {
    cart_id:"$maskedCartId"
  }) {
    order {
      order_id
    }
  }
}

[ghost]

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