Azure Blob Storage support

PBM's object storage support currently includes AWS S3, Minio (with it's S3 API), and GCP Cloud Storage (with it's S3 API).

Let's add support for Azure Blob Storage too.

https://docs.microsoft.com/en-us/rest/api/storageservices/blob-service-rest-api

https://github.com/azure/azure-storage-blob-go/

As it is not S3-compatible the storage configuration sub-section will have to be something other than "storage.s3". "storage.azure" seems the best to me so far.