Add support for CDN auth

[oureveryday]

Some Steam CDNs in China requires a CDN auth to download.

[yaakov-h]

This looks like it just passes null around. Where does the actual auth happen?

[oureveryday]

You need to call CMsgClientGetCDNAuthTokenResponse and get the CDN auth.
Only in Chinese CDNs it will return a non-empty CDN auth string. (Other CDNs will return an empty string)
And if you passed an empty/invaild CDN auth string to Chinese CDN, it will return a 403 error.

[xPaw]

For reference, CDN auth code was removed in #1022.

[oureveryday]

You still need to use a CDN auth to download in Chinese CDNs.

[xPaw]

I know, I am providing a reference as to where it was removed, and you can take a look at how it was implemented before.

You should add a xmldoc for the auth argument and how to get one (GetCDNAuthToken).