Add graphql sub module.

Closes #3423901

modules/custom_field_graphql/src/Plugin/GraphQLCompose/FieldType/CustomFieldEntityReference.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\custom_field_graphql\Plugin\GraphQLCompose\FieldType;
+
+use Drupal\Core\Field\FieldItemInterface;
+use Drupal\graphql\GraphQL\Execution\FieldContext;
+use Drupal\graphql_compose\Plugin\GraphQL\DataProducer\FieldProducerItemInterface;
+use Drupal\graphql_compose\Plugin\GraphQL\DataProducer\FieldProducerTrait;
+use Drupal\graphql_compose\Plugin\GraphQLCompose\GraphQLComposeFieldTypeBase;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * {@inheritdoc}
+ *
+ * @GraphQLComposeFieldType(
+ *   id = "custom_field_entity_reference",
+ * )
+ */
+class CustomFieldEntityReference extends GraphQLComposeFieldTypeBase implements FieldProducerItemInterface {
+
+  use FieldProducerTrait;
+
+  /**
+   * The entity repository.
+   *
+   * @var \Drupal\Core\Entity\EntityRepositoryInterface
+   */
+  protected $entityRepository;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    $instance = parent::create($container, $configuration, $plugin_id, $plugin_definition);
+    $instance->entityRepository = $container->get('entity.repository');
+    return $instance;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function resolveFieldItem(FieldItemInterface $item, FieldContext $context) {
+    if (!$item->entity) {
+      return NULL;
+    }
+
+    $entity = $item->entity;
+    $context->addCacheableDependency($entity);
+
+    return $entity;
+  }
+
+}