Allow custom URL() to be used for image fields

Problem/Motivation

When using viewField() to render an image field it would be nice to be able to pass a Url() instance. It makes sense to limit the allowed values in the UI and the setting form, but it would be helpful to be able to programatically render the field with a derived URL that is not 'content' or 'file'

Steps to reproduce

Proposed resolution

Add something like the below to viewElements() in ImageFormatter.php:

    elseif ($image_link_setting instanceof Url) {
      $url = $image_link_setting;
    }

Remaining tasks

- Add the code
- Write the unit tests

User interface changes

N/A

API changes

Data model changes

None.

Release notes snippet

Url() instances can now be programmatically passed to Image Fields when using viewField(). Example:

   $entityTypeManager = \Drupal::service('entity_type.manager');
   $entityTypeManager->getViewBuilder($entity_type);

    $build = $view_builder->viewField($user->field_image, [
      'label' => 'hidden',
      'settings' => [
        'image_style' => 'large',
        'image_link' => new Url('custom.user.route', ['user' => $user->id()]),
      ],
    ]);