Added a duplicate UI

Entities using Entity API permission and route providers can now have a duplicate form.
The entity needs to define a "duplicate" form and a "duplicate-form" link template.
The duplicate form should implement EntityDuplicateFormTrait and call $this->postSave() after saving the entity, to allow the EntityDuplicateEvent to be fired.

Example from tests:

 *   handlers = {
 *     "form" = {
 *       "add" = "\Drupal\entity_module_test\Form\EnhancedEntityForm",
 *       "edit" = "\Drupal\entity_module_test\Form\EnhancedEntityForm",
 *       "duplicate" = "\Drupal\entity_module_test\Form\EnhancedEntityForm",
 *       "delete" = "\Drupal\Core\Entity\EntityDeleteForm",
 *     },
 *   },
 *   links = {
 *     "add-page" = "/entity_test_enhanced/add",
 *     "add-form" = "/entity_test_enhanced/add/{type}",
 *     "edit-form" = "/entity_test_enhanced/{entity_test_enhanced}/edit",
 *     "duplicate-form" = "/entity_test_enhanced/{entity_test_enhanced}/duplicate",
 *   }

And in EnhancedEntityForm:

class EnhancedEntityForm extends ContentEntityForm {

  use EntityDuplicateFormTrait;

  /**
   * {@inheritdoc}
   */
  public function save(array $form, FormStateInterface $form_state) {
    $this->entity->save();
    $this->postSave($this->entity, $this->operation);

    $this->messenger()->addMessage($this->t('Saved the %label enhanced entity.', ['%label' => $this->entity->label()]));
    $form_state->setRedirect('entity.entity_test_enhanced.collection');
  }

}

Note that if the duplicated entity is a bundle entity, its fields and (form/view) displays will be automatically duplicated as well.