magento-test/app/code/Magento/ConfigurableProduct/etc/extension_attributes.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Api/etc/extension_attributes.xsd">
    <extension_attributes for="Magento\Catalog\Api\Data\ProductInterface">
        <attribute code="configurable_product_options" type="Magento\ConfigurableProduct\Api\Data\OptionInterface[]" />
        <attribute code="configurable_product_links" type="int[]" />
    </extension_attributes>
    <extension_attributes for="Magento\Quote\Api\Data\ProductOptionInterface">
        <attribute code="configurable_item_options" type="Magento\ConfigurableProduct\Api\Data\ConfigurableItemOptionValueInterface[]" />
    </extension_attributes>
    <extension_attributes for="Magento\Catalog\Api\Data\ProductOptionInterface">
        <attribute code="configurable_item_options" type="Magento\ConfigurableProduct\Api\Data\ConfigurableItemOptionValueInterface[]" />
    </extension_attributes>
</config>