Recipient Type Reference does not work with Computed fields

Created a new MR to support computed fields.

Steps for testing:
1. Create a new computed field of the 'email' type for the user entity type and its callback.
e.g.

/**
 * Implements hook_entity_base_field_info().
 */
function {MODULE}_entity_base_field_info(EntityTypeInterface $entity_type) {
  $fields = [];

  switch ($entity_type->id()) {
    case 'user':
      $fields['computed'] = \Drupal\Core\Field\BaseFieldDefinition::create('email')
        ->setLabel(t('Computed mail'))
        ->setCardinality(1)
        ->setComputed(TRUE)
        ->setCustomStorage(TRUE)
        ->setClass('\Drupal\{MODULE}\Field\{MODULE}EmailField');
      break;

  }

  return $fields;
}

2. Create a new Contact form on the /admin/structure/contact/add page and add a new user reference field.
3. Create a new Contact email on the /admin/structure/contact/emails/add/add page and select the contact form from the above.
4. Select the 'The value of a specific field in an entity reference' value for the 'Recipient type' field.

Results: you should see your computed field. Creating a new contact message should send an e-mail to the e-mail address from the computed field.

Merged the MR.

Changes will be added to the next release.

Automatically closed - issue fixed for 2 weeks with no activity.