Magento – Magento 2 extension attribute issue for Magento\Quote\Api\Data\AddressInterface

I figured it by myself... extension attributes are for more complex info.

For this particular case where it is a "simple" custom attribute (of type string), the way for updating via API is just to pass it inside of custom attributes array:

    $headers = array(
        'Content-Type: application/json',
        'Authorization: Bearer ' . $token
    );
    $data = array(
        'product' => array(
            'custom_attributes' => array(
                '0' => array(
                    'attribute_code' => 'needs_sync',
                    'value' => '0'
                )
            )
        )
    );
    $data = json_encode($data);
    $requestUrl = $this->_baseUrl . '/index.php/rest/V1/products/' . $sku;
    $ch = curl_init($requestUrl);
    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "PUT");
    curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
    curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    $result = curl_exec($ch);

You have to setExtensionAttributes manually.

Example:

CustomerRepositoryInterfacePlugin.php

 use Magento\Customer\Api\CustomerRepositoryInterface;
 use Magento\Customer\Api\Data\CustomerExtensionFactory;


class CustomerRepositoryInterfacePlugin
{

[...]
public function __construct(\Psr\Log\LoggerInterface $logger,
                            CustomerExtensionFactory $customerExtensionFactory)
{
    $this->logger = $logger;
    $this->customerExtensionFactory = $customerExtensionFactory;
}

public function afterGetById(CustomerRepositoryInterface $subject,    \Magento\Customer\Api\Data\CustomerInterface $result)
 {
    $extension = $result->getExtensionAttributes() ? $result->getExtensionAttributes() : $this->customerExtensionFactory->create();

    $extension->setCustomAttribute("Custom Attribute");

    $result->setExtensionAttributes($extension);
    return $result;
}
}

di.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
      <type name="Magento\Customer\Api\CustomerRepositoryInterface">
         <plugin name="Namespace-Module" type="Namespace\Module\Plugin\CustomerRepositoryInterfacePlugin" sortOrder="5"/>
     </type>
</config>