Add field UI to shipping methods

Shipping methods are fieldables, there's simply no UI for adding fields.

And what do we need to do to add that missing UI?

Right now the only way I could add fields to shipping methods is by doing all sorts of update hooks which seem really counterproductive compared to clicking around a few Drupal field forms:

/**
 * Implements hook_install().
 */
function sap_b1_install() {
  $entity_type = \Drupal::entityTypeManager()->getDefinition('commerce_shipping_method');
  \Drupal::entityDefinitionUpdateManager()->updateEntityType($entity_type);
}

/**
 * Implements hook_uninstall().
 */
function sap_b1_uninstall() {
  $entity_type = \Drupal::entityTypeManager()->getDefinition('commerce_shipping_method');
  \Drupal::entityDefinitionUpdateManager()->updateEntityType($entity_type);
}

/**
 * Add sap_shipping_type field to commerce_shipping_method.
 */
function sap_b1_update_8001(&$sandbox) {
  $sap_shipping_type = BaseFieldDefinition::create('string')
      ->setLabel(t('SAP ShippingType Code'))
      ->setCardinality(1)
      ->setDisplayConfigurable('form', TRUE)
      ->setDisplayConfigurable('view', FALSE)
      ->setDefaultValue(NULL);

  \Drupal::entityDefinitionUpdateManager()
    ->installFieldStorageDefinition(
      'sap_shipping_type',
      'commerce_shipping_method',
      'sap_b1',
      $sap_shipping_type);
}

/**
 * Implements hook_entity_base_field_info().
 */
function sap_b1_entity_base_field_info(EntityTypeInterface $entity_type) {
 $fields = [];

 if ($entity_type->id() == 'commerce_shipping_method') {
   $fields['sap_shipping_type'] = BaseFieldDefinition::create('string')
     ->setLabel(t('SAP ShippingType Code'))
     ->setCardinality(1)
     ->setDisplayConfigurable('form', TRUE)
     ->setDisplayOptions('form', [
       'type' => 'string_textfield',
       'weight' => 4,
     ])
     ->setDisplayConfigurable('view', FALSE)
     ->setDefaultValue(NULL);
 }

 return $fields;
}