Using callable as allowed_values_function causes error in field UI

Problem/Motivation

When creating list fields it is possible to use functions to define the allowed values for the list. This can either be done using a fixed list configured while creating the field in the UI or by setting the property allowed_values_function in the field storage configuration.

The property not only allows setting a procedural function but allows developers to use callbacks defined in classes (see #2982949):

This allows using something like

<?php
  /**
   * Implements hook_entity_field_storage_info_alter().
   */
  #[Hook('entity_field_storage_info_alter')]
  public function entityFieldStorageInfoAlter(&$fields, EntityTypeInterface $entity_type): void {
    if (($entity_type->id() === 'node') && isset($fields['field_test_alllowed_values'])) {
      $fields['field_test_allowed_values']->setSetting('allowed_values_function', [MyOptions::class, 'allowedValues']);
    }
}

Unfortunately this results in an error when trying to edit the field in the UI:

TypeError: Drupal\Component\Utility\Html::escape(): Argument #1 ($text) must be of type string, array given, called in /var/www/html/docroot/core/lib/Drupal/Component/Render/FormattableMarkup.php on line 238 in Drupal\Component\Utility\Html::escape() (line 433 of core/lib/Drupal/Component/Utility/Html.php).

When printing information about the allowed_values_function in ListItemBase::storageSettingsForm(), it is always assumed that the value for “allowed_values_function” is a string. Since it is allowed to pass the callable as array this assumption is wrong.

Steps to reproduce

* add a field of type "List (text)" to a content type
* alter the field storage info of this field in a custom module and set the allowed_values_function as shown above
* open the field settings in the admin UI

Proposed resolution

* update ListItemBase::storageSettingsForm() to handle cases where allowed_values_function is an array