Save and load workflows in the new format, update API

[MAILPOET-4523]

mailpoet/lib/Automation/Engine/Data/NextStep.php

@@ -0,0 +1,28 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Data;
+
+class NextStep {
+  /** @var string */
+  protected $id;
+
+  public function __construct(
+    string $id
+  ) {
+    $this->id = $id;
+  }
+
+  public function getId(): string {
+    return $this->id;
+  }
+
+  public function toArray(): array {
+    return [
+      'id' => $this->id,
+    ];
+  }
+
+  public static function fromArray(array $data): self {
+    return new self($data['id']);
+  }
+}