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/Storage/WorkflowStorage.php

@@ -3,6 +3,7 @@
 namespace MailPoet\Automation\Engine\Storage;
 
 use MailPoet\Automation\Engine\Exceptions;
+use MailPoet\Automation\Engine\Workflows\Trigger;
 use MailPoet\Automation\Engine\Workflows\Workflow;
 use wpdb;
 
@@ -45,4 +46,20 @@ class WorkflowStorage {
     }
     return array_unique($triggerKeys);
   }
+
+  /** @return Workflow[] */
+  public function getActiveWorkflowsByTrigger(Trigger $trigger): array {
+    $query = strval(
+      $this->wpdb->prepare(
+        "SELECT * FROM $this->table WHERE status = %s AND trigger_keys LIKE %s",
+        Workflow::STATUS_ACTIVE,
+        '%' . $this->wpdb->esc_like($trigger->getKey()) . '%'
+      )
+    );
+
+    $data = $this->wpdb->get_results($query, ARRAY_A);
+    return array_map(function (array $workflowData) {
+      return Workflow::fromArray($workflowData);
+    }, (array)$data);
+  }
 }