I have url where I need to download file and convert to base64 string

I wrote this code to do this

```
private string GetImageAsBase64(string url)
    {
        using (var client = new WebClient())
        {
            var bytes = client.DownloadData(url);
            var base64String = Convert.ToBase64String(bytes);
            return base64String;
        }
    }

```

How I can make it async to use returning type `Task<string>`?