New hook_entity_form_mode_alter() added

hook_entity_form_mode_alter() hook was added to allow modules to override form mode based on some conditions. For example, different form modes can be applied based on user role.

Example:

function hook_entity_form_mode_alter(&$form_mode, Drupal\Core\Entity\EntityInterface $entity) {
  // Change the form mode for users with Administrator role.
  if ($entity->getEntityTypeId() == 'user' && $entity->hasRole('administrator')) {
    $form_mode = 'my_custom_form_mode';
  }
}