Add extension point to WebAPI

[vrann]

• Сreate Request Processor Interface
• inject pool of Request Processors to REST \Magento\Webapi\Controller\Rest::dispatch
• implement Async Request Processors Processor matcher which will handle async requests
• implement simple async Request processor interface (in a new module) which responds with 202 Accepted and hardcoded message
• write api-functional tests

[paliarush]

Pool declaration example:

<?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:ObjectManager/etc/config.xsd">
    <type name="Magento\Webapi\Controller\Rest\RequestProcessorPool">
        <arguments>
            <argument name="requestProcessors" xsi:type="array">
                <item name="schemaRequestProcessor" xsi:type="array">
                    <item name="type" xsi:type="object">Magento\Webapi\Controller\Rest\SchemaRequestProcessor</item>
                    <item name="sortOrder" xsi:type="string">10</item>
                </item>
                <item name="synchronousRequestProcessor" xsi:type="array">
                    <item name="type" xsi:type="object">Magento\Webapi\Controller\Rest\SynchronousRequestProcessor</item>
                    <item name="sortOrder" xsi:type="string">20</item>
                </item>
            </argument>
        </arguments>
    </type>
</config>

Request processor interface example:

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Webapi\Controller\Rest;

interface RequestProcessorInterface
{
    /**
     * @param \Magento\Framework\Webapi\Rest\Request $request
     * @return bool
     */
    public function canProcess(\Magento\Framework\Webapi\Rest\Request $request);

    /**
     * @param \Magento\Framework\Webapi\Rest\Request $request
     * @return void
     * @throws \Magento\Framework\Exception\AuthorizationException
     * @throws \Magento\Framework\Exception\InputException
     * @throws \Magento\Framework\Webapi\Exception
     */
    public function process(\Magento\Framework\Webapi\Rest\Request $request);
}

Example implementation of RequestProcessorPool:

    /**
     * @param \Magento\Framework\Webapi\Rest\Request $request
     * @return \Magento\Webapi\Controller\Rest\RequestProcessorInterface
     * @throws \Magento\Framework\Exception\NotFoundException
     */
    public function getRequestProcessor(\Magento\Framework\Webapi\Rest\Request $request)
    {
        foreach ($this->requestProcessors as $requestProcessor) {
            if ($requestProcessor->canProcess($request)) {
                return $requestProcessor;
            }
        }
        throw new \Magento\Framework\Exception\NotFoundException(__('Specified request does not match any route.'));
    }

[naydav]

I described my concerns about these interfaces in magento/bulk-api-ce#1 (review)