Add a runtime for the symfony/runtime component.

[GeraudBourdin]

Symfony runtime for SonataPage

Add a SonataPage runtime to avoid custom configuration in the public/index.php

use the configuration above for testing this.

composer require symfony/runtime

public/index.php :

    use App\Kernel;
    
    require_once dirname(__DIR__).'/vendor/autoload_runtime.php';

    return function (array $context) {
        return new Kernel($context['APP_ENV'], (bool) $context['APP_DEBUG']);
    };

composer.json :

{
    // ... other composer configs ...
    "extra": {
        // ... other composer configs ...
        "runtime": {
            "class": "Sonata\\PageBundle\\Runtime\\SonataPageRuntime",
            "multisite": "host_with_path_by_locale"
        }
    }
}

multisite is not manatory, by default host_with_path_by_locale is selected

Documentation used : https://symfony.com/doc/current/components/runtime.html

Closes #1604

Changelog

### Added
- Some `Sonata\PageBundle\Runtime\SonataPageRuntime` in order to use the symfony runtime component

### Changed

### Deprecated

### Removed

### Fixed

### Security

[VincentLanglet]

Please target the stable version 4.x , or is it a BC break ?

[GeraudBourdin]

Please target the stable version 4.x , or is it a BC break ?
Done.