[TF2] Allow VScript scripts to add to the download list

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Currently, only native/SourceMod plugins can add to the download list, which means that if I would want to make an add-on that requires custom assets, I would either need to use a SourceMod plugin to add the assets to the download list, or pack it with the map.

I propose adding a function like AddToDownloadList(string filepath) or a hook (OnScriptHook_ClientConnected?) when a player connects which returns an array of strings containing the filepaths to download.

This isn't limited to TF2, but it's where I wish the feature to be added the most.

[Hex4dec]

I've implemented this in https://github.com/ValveSoftware/source-sdk-2013/pull/1706