Add managed identities (temporary credentials) support for Azure Blob storage

Go Cloud v0.21.0 was just released and now supports this: https://github.com/google/go-cloud/releases/tag/v0.21.0 via https://github.com/google/go-cloud/issues/2870.

More info: https://docs.microsoft.com/en-us/azure/active-directory/managed-identities-azure-resources/overview