Apply PO token and Visitor data

[clytras]

Could we make it so we can apply our PO token and Visitor Data? There are a couple of ways to retrieve those values both in backend and in browsers, so it would be nice if we have the option to give and apply those values if we have them and just fallback back to iOS client by default if the experiment is detected.

~PS: I am getting 403 errors after dowloading some chunks using the latest version and it was working a couple days ago. I think that if I can apply PO/VD tokens, such errors might be elliminated.~