S3 backend: provide client side encryption

[nob13]

The s3 backend is very nice and easy to deploy huge amounts of data. However there is only a switch for server side encryption which is fairly useless as you have to fully trust Amazon on it, and some customers usually don't do.

Is it possible to add client side encryption? There seem to be already libraries offering this

[dmp42]

Thanks @nob13

I'm adding this request to the new golang registry home.

This is probably not going to happen for the current python registry (here).