Create a commercetools cart, add line items with custom fields, and transition it to an order using the Carts and Orders API

Steps

1. Authenticate with commercetools using the client credentials OAuth flow to obtain an access token scoped to the manage_orders scope
2. Create a cart by posting to the carts endpoint with the required currency, customer email or customerId, and any desired shipping address
3. Add line items to the cart using the addLineItem update action, specifying the productId and variantId; if the line items need custom fields, first ensure a custom type is assigned to the cart or line item using the setLineItemCustomType update action
4. Set custom field values on the line items using the setLineItemCustomField update action with the field name and value matching the type definition
5. Create an order from the cart by posting to the orders endpoint with the cartId and the cart version; handle the version conflict error by re-fetching the cart version before retrying
6. Verify the resulting order has the correct state, line items, and custom fields by fetching the order by ID