Add a generic trait for logic to convert references into Urls in LinkWidget

Initial approach moving the logic to a trait in the core namespace, so it could be used by Link module and others , avoiding to add the dependency on Link module.

A good candidate to make use of this trait would be 📌 Provide a NavigationLinkBlock Plugin and use Help as an usage example Needs review once that MR gets in.

Final trait placement and/or Name is open for discussion, but this initial MR is ready for review.

Move seems straight forward and non disruptive since function names are same.

I drafted a simple CR to announce the new trait here https://www.drupal.org/node/3455776 →

In an ideal world...

if ($entity_type == 'node' && $entity = \Drupal::entityTypeManager()
        ->getStorage($entity_type)
        ->load($entity_id)) {
        $displayable_string = EntityAutocomplete::getEntityLabels([$entity]);
      }

the validation would some how allow us to use injectable code - a service or something - but it doesn't so this is okay and allows this code to be shared. See how \Drupal\Core\Form\FormValidator::doValidateForm() works.

Think about this another way - should we introduce another render element - or could we move this to \Drupal\Core\Render\Element\Url and have it as an optional thing?

Looking at how we need to manipulate the value we should add a new form element that accepts a Url object as the value.