Implement downloading and decrypting depots, manifests, chunks

[mjsir911]

I basically want to rewrite DepotDownloader in go, and that requires all the DepotManifest APIs

Related codes:

• https://github.com/SteamRE/SteamKit/blob/master/SteamKit2/SteamKit2/Steam/CDNClient.cs#L334
• https://github.com/SteamRE/SteamKit/blob/master/SteamKit2/SteamKit2/Types/DepotManifest.cs#L218

The protobufs are there, but other than that nothing else really

[mjsir911]

In my travels I've found that #21 & #100 are closely integrated with this issue, since to find the urls for manifests you need to be able to parse the downloaded .acf/.vdf files