Refactor CDN assets URL to separate service

[MAILPOET-3076]

lib/Util/CdnAssetUrl.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace MailPoet\Util;
+
+class CdnAssetUrl {
+  const CDN_URL = 'https://ps.w.org/mailpoet/';
+  /** @var string */
+  private $baseUrl;
+
+  public function __construct(string $baseUrl) {
+    $this->baseUrl = $baseUrl;
+  }
+
+  public function generateCdnUrl($path) {
+    $useCdn = defined('MAILPOET_USE_CDN') ? MAILPOET_USE_CDN : true;
+    return ($useCdn ? self::CDN_URL : $this->baseUrl . '/plugin_repository/') . "assets/$path";
+  }
+}