Allow setting a default target type

Currently it's possible to achieve this with some somewhat hacky alter code

 function my_module_field_widget_single_element_dynamic_entity_reference_default_form_alter(array &$element, FormStateInterface $formState, array $context): void {
    $items = $context['items'] ?? NULL;
    if (!$items instanceof DynamicEntityReferenceFieldItemList) {
      return;
    }

    // If the element already has an entity, bail.
    if (!empty($element['target_id']['#default_value'])) {
      return;
    }

    // Default to node.
    $targetType = 'node';
    $fieldSettings = $items->getFieldDefinition()->getSettings();
    $element['target_type']['#default_value'] = $targetType;
    $element['target_id']['#target_type'] = $targetType;
    $element['target_id']['#selection_handler'] = $fieldSettings[$targetType]['handler'];
  }