/downloads/protected/ folder and internal redirect

[amitu]

We will have ftd.internal-redirect and ft_sdk::internal_redirect() responses to support internal redirect, meaning the URL will remain what was requested, and we will actually return 200 (not redirect status code), and serve the content of the file (or http request) that is pointed as the Location.

Files in /downloads/protected/ can only be downloaded via internal redirect.

This can be used to implement password protected or payment protected etc download of static files.