Adds method to return subscriber fields

lib/API/MP/v1/API.php

@@ -1,7 +1,35 @@
 <?php
 namespace MailPoet\API\MP\v1;
 
+use MailPoet\Models\CustomField;
+
 if(!defined('ABSPATH')) exit;
 
 class API {
+  function getSubscriberFields() {
+    $data = array(
+      array(
+        'id' => 'email',
+        'name' => __('Email', 'mailpoet')
+      ),
+      array(
+        'id' => 'first_name',
+        'name' => __('First name', 'mailpoet')
+      ),
+      array(
+        'id' => 'last_name',
+        'name' => __('Last name', 'mailpoet')
+      )
+    );
+
+    $custom_fields = CustomField::selectMany(array('id', 'name'))->findMany();
+    foreach($custom_fields as $custom_field) {
+      $data[] = array(
+        'id' => 'cf_' . $custom_field->id,
+        'name' => $custom_field->name
+      );
+    }
+
+    return $data;
+  }
 }