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).