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_SimpleSchedule.php

@@ -0,0 +1,81 @@
+<?php
+
+/**
+ * Class ActionScheduler_SimpleSchedule
+ */
+class ActionScheduler_SimpleSchedule extends ActionScheduler_Abstract_Schedule {
+
+	/**
+	 * Deprecated property @see $this->__wakeup() for details.
+	 *
+	 * @var null|DateTime
+	 */
+	private $timestamp = null;
+
+	/**
+	 * Calculate when this schedule should start after a given date & time using
+	 * the number of seconds between recurrences.
+	 *
+	 * @param DateTime $after Timestamp.
+	 *
+	 * @return DateTime|null
+	 */
+	public function calculate_next( DateTime $after ) {
+		return null;
+	}
+
+	/**
+	 * Schedule is not recurring.
+	 *
+	 * @return bool
+	 */
+	public function is_recurring() {
+		return false;
+	}
+
+	/**
+	 * Serialize schedule with data required prior to AS 3.0.0
+	 *
+	 * Prior to Action Scheduler 3.0.0, schedules used different property names to refer
+	 * to equivalent data. For example, ActionScheduler_IntervalSchedule::start_timestamp
+	 * was the same as ActionScheduler_SimpleSchedule::timestamp. Action Scheduler 3.0.0
+	 * aligned properties and property names for better inheritance. To guard against the
+	 * scheduled date for single actions always being seen as "now" if downgrading to
+	 * Action Scheduler < 3.0.0, we need to also store the data with the old property names
+	 * so if it's unserialized in AS < 3.0, the schedule doesn't end up with a null recurrence.
+	 *
+	 * @return array
+	 */
+	public function __sleep() {
+
+		$sleep_params = parent::__sleep();
+
+		$this->timestamp = $this->scheduled_timestamp;
+
+		return array_merge(
+			$sleep_params,
+			array(
+				'timestamp',
+			)
+		);
+	}
+
+	/**
+	 * Unserialize recurring schedules serialized/stored prior to AS 3.0.0
+	 *
+	 * Prior to Action Scheduler 3.0.0, schedules used different property names to refer
+	 * to equivalent data. For example, ActionScheduler_IntervalSchedule::start_timestamp
+	 * was the same as ActionScheduler_SimpleSchedule::timestamp. Action Scheduler 3.0.0
+	 * aligned properties and property names for better inheritance. To maintain backward
+	 * compatibility with schedules serialized and stored prior to 3.0, we need to correctly
+	 * map the old property names with matching visibility.
+	 */
+	public function __wakeup() {
+
+		if ( is_null( $this->scheduled_timestamp ) && ! is_null( $this->timestamp ) ) {
+			$this->scheduled_timestamp = $this->timestamp;
+			unset( $this->timestamp );
+		}
+		parent::__wakeup();
+	}
+}