Feature: #69918 - Add PSR-7-based dispatching for Backend Modules

See Issue #69918

Description

Built on the PSR-7 principle and the routing concepts, it is now possible to register backend modules which are dispatched to a callable string instead of pointing to an index.php file in EXT:myextension/Modules/MyModule/index.php.

The method which is called, receives a PSR-compatible request and response object and must return a response object which is outputted to the browser.

An example registration uses the option routeTarget to resolve the method to be called when rendering the module:

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addModule(
        'web',
        'layout',
        'top',
        '',
        array(
                'routeTarget' => \TYPO3\CMS\Backend\Controller\PageLayoutController::class . '::mainAction',
                'access' => 'user,group',
                'name' => 'web_layout',
                'labels' => array(
                        'tabs_images' => array(
                                'tab' => 'EXT:backend/Resources/Public/Icons/module-page.svg',
                        ),
                        'll_ref' => 'LLL:EXT:backend/Resources/Private/Language/locallang_mod.xlf',
                ),
        )
);