Add default attributes for native lazy loading

[alexander-schranz]

Native lazy loading

Browsers today have native support for lazy-loading.

You can do the following:

{{ get_image(image, {
    src: '800x',
    loading: 'lazy',
}) }}

This pull requests add default attributes so you can add specific attributes to all your get_image tagged calls by using the following service definition:

services:
    Sulu\Twig\Extensions\ImageExtension:
        arguments:
            - null
            - { loading: 'lazy' }

[luca-rath]

thank you! can be merged now :)