Add basic request, response, and endpoint definitions for API

[MAILPOET-4135]

mailpoet/lib/Automation/API/Response.php

@@ -0,0 +1,14 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\API;
+
+use WP_REST_Response;
+
+class Response extends WP_REST_Response {
+  public function __construct(
+    array $data = null,
+    int $status = 200
+  ) {
+    parent::__construct(['data' => $data], $status);
+  }
+}