azure: add support for using the secondary endpoint for the download_url

The idea is to allow using Azure's secondary endpoint
(BlobSecondaryEndpoint) to craft the download url the storage used as
public download URL.

This is needed for the integration test where the endpoint used to add
content in an azure-backend objstorage is different from the public URL
that can be used to download the blob directly from azure (azurite in the
context of integration tests).