Access private protected object storage

[ccancellieri]

Hi, I'm proposing to add an option to let ckan instance to work as a proxy to download private data.

Currently if I would like to access to a bucket (not Azure, etc, I'm on Google) I've to set it as public.

If file object is protected I get:

<Error>
<Code>AccessDenied</Code>
<Message>Access denied.</Message>
<Details>Anonymous caller does not have storage.objects.get access to the Google Cloud Storage object.</Details>
</Error>