Credentials for "system"-S3 buckets

[normanrz]

Detailed Description

Add a configuration option for tuples of (bucket_name, endpoint_url, access_key_id, secret_access_key, ...) for buckets that Webknossos manages itself. These credentials must not be used for the user-facing explore functionality. This probably belongs in the datastore.

Context

We want to natively support storing data in S3-like object storage. In contrast to the filesytems, the access control is managed via explicit credentials and not user accounts under which the application is running. Therefore, we need a way to specify these credentials in Webknossos.

Environment variables would be an alternative to configs. However, I foresee that we will want to use multiple buckets that might have different credentials. Also, env vars might be accidentially picked up by the explore functionality, which we must prevent.