Rename namespace with defs for 3rd-party integrations to "Integration"

[MAILPOET-4659]

mailpoet/lib/Automation/Engine/Integration/Trigger.php

@@ -0,0 +1,11 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Integration;
+
+use MailPoet\Automation\Engine\Data\StepRunArgs;
+
+interface Trigger extends Step {
+  public function registerHooks(): void;
+
+  public function isTriggeredBy(StepRunArgs $args): bool;
+}