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

[MAILPOET-4659]

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

@@ -0,0 +1,20 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Integration;
+
+use MailPoet\Automation\Engine\Data\Subject as SubjectData;
+use MailPoet\Validator\Schema\ObjectSchema;
+
+/**
+ * @template-covariant T of Payload
+ */
+interface Subject {
+  public function getKey(): string;
+
+  public function getName(): string;
+
+  public function getArgsSchema(): ObjectSchema;
+
+  /** @return T */
+  public function getPayload(SubjectData $subjectData): Payload;
+}