chore(06-02): download and include Action Scheduler library

- Downloaded Action Scheduler 3.9.3 from GitHub
- Placed in vendor/action-scheduler/ directory
- Included in main plugin file before other code for proper initialization
- Library will auto-initialize itself when required

vendor/action-scheduler/classes/schedules/ActionScheduler_NullSchedule.php

@@ -0,0 +1,39 @@
+<?php
+
+/**
+ * Class ActionScheduler_NullSchedule
+ */
+class ActionScheduler_NullSchedule extends ActionScheduler_SimpleSchedule {
+
+	/**
+	 * DateTime instance.
+	 *
+	 * @var DateTime|null
+	 */
+	protected $scheduled_date;
+
+	/**
+	 * Make the $date param optional and default to null.
+	 *
+	 * @param null|DateTime $date The date & time to run the action.
+	 */
+	public function __construct( ?DateTime $date = null ) {
+		$this->scheduled_date = null;
+	}
+
+	/**
+	 * This schedule has no scheduled DateTime, so we need to override the parent __sleep().
+	 *
+	 * @return array
+	 */
+	public function __sleep() {
+		return array();
+	}
+
+	/**
+	 * Wakeup.
+	 */
+	public function __wakeup() {
+		$this->scheduled_date = null;
+	}
+}