Export multiple collections/records

Sometimes you have the permission to read some collections but not the bucket.
In that case it would be great to be able to list all the collections that you want to export to be able to export them all even when you cannot read the bucket's collection list.

Start by tracing the export flow in kinto-wizard and how it lists bucket collections. Determine how an explicit list of collections or records could be accepted without reading the bucket's collection list; done means permitted collections can be exported together despite that restriction.

Written by the indexing model from the issue text.