Add custom fields to the response

[MAILPOET-2653]

lib/Subscribers/SubscriberCustomFieldRepository.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace MailPoet\Subscribers;
+
+use MailPoet\Doctrine\Repository;
+use MailPoet\Entities\SubscriberCustomFieldEntity;
+
+/**
+ * @extends Repository<SubscriberCustomFieldEntity>
+ */
+class SubscriberCustomFieldRepository extends Repository {
+  protected function getEntityClassName() {
+    return SubscriberCustomFieldEntity::class;
+  }
+}