Add user agent string

Source/QuestPDF.Examples/LoremPicsumExample.cs

@@ -23,6 +23,7 @@ namespace QuestPDF.Examples
                 url += "?grayscale";
 
             using var client = new WebClient();
+            client.Headers.Add("user-agent", "QuestPDF/1.0 Unit Testing");
             var response = client.DownloadData(url);
             container.Image(response);
         }