Define URL utility method to check if url is using https

[MAILPOET-4702]

mailpoet/lib/Util/Url.php

@@ -69,4 +69,8 @@ class Url {
     }
     exit();
   }
+
+  public function isUsingHttps(string $url): bool {
+    return wp_parse_url($url, PHP_URL_SCHEME) === 'https';
+  }
 }