Proxied download restriction

[myl7]

Currently in proxied download, url param has no limitation. Someone may access urls like https://drive.spencerwoo.com/api/proxy?url=https%3A%2F%2Fgoogle.github.io%2Fstyleguide%2Fcppguide.html to use the api as a proxy, which does not seem to be expected.
To ensure url is valid, maybe we can check if it is an index api raw=true redirection.

And for people who do not use it, as proxied download will consume Vercel bandwidth limitation, maybe an option to turn it off is required.

[spencerwooo]

proxied download now tries to match the directLink defined in api.json before downloading (as i believe sharepoint's direct link urls are customised)