Feature: #88871 - Handle middleware handler in RequestFactory

Description

Guzzle offers the possibility to register custom middleware handlers during the client initialization. With this feature it is now possible to define those custom handlers in $GLOBALS['TYPO3_CONF_VARS']['HTTP']['handler'] as an array. The \TYPO3\CMS\Core\Http\RequestFactory builds a handler stack based on the $GLOBALS['TYPO3_CONF_VARS']['HTTP']['handler'] array and injects it into the created client.

Impact

The default handler stack (guzzle defaults) will be extended and not overwritten.

Example:

# Add custom middleware to default Guzzle handler stack
$GLOBALS['TYPO3_CONF_VARS']['HTTP']['handler'][] =
   (\TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\ACME\Middleware\Guzzle\CustomMiddleware::class))->handler();
$GLOBALS['TYPO3_CONF_VARS']['HTTP']['handler'][] =
   (\TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\ACME\Middleware\Guzzle\SecondCustomMiddleware::class))->handler();