Move automation engine code under "Engine" namespace

This is to separate the engine itself from "integrations" that will be
built on top of the engine.

[MAILPOET-4136]

mailpoet/lib/Automation/Engine/API/Endpoints/WorkflowsEndpoint.php

@@ -0,0 +1,13 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\API\Endpoints;
+
+use MailPoet\Automation\Engine\API\Endpoint;
+use MailPoet\Automation\Engine\API\Request;
+use MailPoet\Automation\Engine\API\Response;
+
+class WorkflowsEndpoint extends Endpoint {
+  public function get(Request $request): Response {
+    return new Response(['message' => 'Hello world.']);
+  }
+}