Optional support for AssetExtension for ImageTwigExtension

[alexander-schranz]

Currently the ImageExtension does not have support for asset function this should be introduce by optional injecting the AssetExtension of Symfony Twig Bridge.

E.g.:

services:
      Sulu\Twig\Extensions\ImageTwigExtension:
          arguments:
               - 'placeholders'
               - '@?twig.extension.assets'

public function __construct(
    ?string $placeholderPath = null, 
    ?AssetExtension $assetExtension, 
    ?string $defaultPackageName
) {
     // ...
}

public function getImage($media, $attributes = [], array $sources = [], ?string $packageName = null): string {
     // ...
}