Create a new backend instance authenticating via a connection string.
Intended for the Azurite <https://learn.microsoft.com/azure/storage/common/storage-use-azurite>_
emulator, or any account where a connection string (rather than
account_url + credential) is more convenient.
Full connection string (e.g. from the Azure portal, or for the Azurite emulator).
Target blob container name.
Optional key namespace prefix within the container.
