Add support for expiring download_urls and download endpoint

[davidwessman]

Hello!

We would like to find some way to make expiring urls when using download_url, and check this with some signed parameter in the Uploader.download_response.

As far as I can see that is not possible right now, is there anyway to do it with the current implementation or any tips on how to do it? Would it make sense to add it to the gem?

[janko]

Yes, I would welcome a PR that adds this to the download_endpoint plugin. There is an implementation of expiring URLs in the derivation_endpoint plugin, maybe a similar strategy can be used, though that would require signing.

[davidwessman]

I wrote a PR that uses ActiveSupport::MessageVerifier, would love some feedback 🙂
#708