magento-test/app/code/Magento/Eav/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\Eav\Api\Data\AttributeGroupInterface">
        <attribute code="attribute_group_code" type="string" />
        <attribute code="sort_order" type="string" />
    </extension_attributes>
</config>