add a multiselect for campaign action #1866 for 2.4

Model/Config/Source/CampaignAction.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Ebizmarts\MailChimp\Model\Config\Source;
+
+class CampaignAction implements \Magento\Framework\Option\ArrayInterface
+{
+    private $actions = [
+        ['value' => 'sent', 'label' => 'Sent'],
+        ['value' => 'open', 'label' => 'Open'],
+        ['value' => 'click', 'label' => 'Click']
+    ];
+    public function toOptionArray()
+    {
+        return $this->actions;
+    }
+}