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

[MAILPOET-4659]

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

@@ -0,0 +1,16 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Integration;
+
+use MailPoet\Validator\Schema\ObjectSchema;
+
+interface Step {
+  public function getKey(): string;
+
+  public function getName(): string;
+
+  public function getArgsSchema(): ObjectSchema;
+
+  /** @return string[] */
+  public function getSubjectKeys(): array;
+}