Add optional s3 auth via params

[tholor]

Adding another option to do authentication in download_from_s3()

The default and recommended authentication follows boto3's trajectory of checking for ENV variables, .aws/credentials etc. (see https://boto3.amazonaws.com/v1/documentation/api/latest/guide/credentials.html).

With this PR, there's also the option to pass access_key, secret_access_key and region_name directly as this is needed in some enterprise environments with local s3 deployments.