WebRequestLoader does not support proxy server

Hello,

Currently HttpClient in WebRequestLoader is private,
And WebRequestLoader does not provide any method to support proxy server.
If the program is running behind the proxy, then there is no way to download the asset from web.
Thank you.

example code,
```
proxy = new WebProxy
{
Address = new Uri(proxyUrl),
UseDefaultCredentials = true,
};

httpClientHandler = new HttpClientHandler
{
Proxy = proxy,
};

httpClient = new HttpClient(handler: httpClientHandler, disposeHandler: true);

```