Add support for Enum Field

Problem/Motivation

The Enum Field → module allows you to create List fields (integer/string) that get their allowed values from a PHP enum class. It also provides shortcuts to easily extract enum instances from field items and field item lists, for use in custom code:

$entity->get('field_some_enum')->enum
$entity->get('field_some_enum')->enums()

Proposed resolution

Add support for enum fields.