Add hook and basic handler for automation triggers

For now trigger keys are stored in the workflow as a JSON array.
This is not optimal in case someone has many workflows but as
workflows are user-created it's unlinkely someone will have thousands
of them. We can consider adding a workflow_triggers table as well.

[MAILPOET-4136]

mailpoet/lib/Automation/Engine/Control/TriggerHandler.php

@@ -0,0 +1,46 @@
+<?php declare(strict_types = 1);
+
+namespace MailPoet\Automation\Engine\Control;
+
+use MailPoet\Automation\Engine\Exceptions;
+use MailPoet\Automation\Engine\Hooks;
+use MailPoet\Automation\Engine\Storage\WorkflowStorage;
+use MailPoet\Automation\Engine\WordPress;
+use MailPoet\Automation\Engine\Workflows\Trigger;
+
+class TriggerHandler {
+  /** @var ActionScheduler */
+  private $actionScheduler;
+
+  /** @var WordPress */
+  private $wordPress;
+
+  /** @var WorkflowStorage */
+  private $workflowStorage;
+
+  public function __construct(
+    ActionScheduler $actionScheduler,
+    WordPress $wordPress,
+    WorkflowStorage $workflowStorage
+  ) {
+    $this->actionScheduler = $actionScheduler;
+    $this->wordPress = $wordPress;
+    $this->workflowStorage = $workflowStorage;
+  }
+
+  public function initialize(): void {
+    $this->wordPress->addAction(Hooks::TRIGGER, [$this, 'processTrigger']);
+  }
+
+  public function processTrigger(Trigger $trigger): void {
+    $workflows = $this->workflowStorage->getActiveWorkflowsByTrigger($trigger);
+    foreach ($workflows as $workflow) {
+      $step = $workflow->getTrigger($trigger->getKey());
+      if (!$step) {
+        throw Exceptions::workflowTriggerNotFound($workflow->getId(), $trigger->getKey());
+      }
+
+      // TODO: create new workflow run
+    }
+  }
+}