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/actions/ActionScheduler_FinishedAction.php

@@ -0,0 +1,21 @@
+<?php
+
+/**
+ * Class ActionScheduler_FinishedAction
+ */
+class ActionScheduler_FinishedAction extends ActionScheduler_Action {
+
+	/**
+	 * Execute action.
+	 */
+	public function execute() {
+		// don't execute.
+	}
+
+	/**
+	 * Get finished state.
+	 */
+	public function is_finished() {
+		return true;
+	}
+}