Enhancement: Allow use of SAS token to authenticate to Azure

Previously restic only supported AccountKeys to authenticate to Azure
storage accounts, which necessitates giving a significant amount of
access.

We added support for Azure SAS tokens which are a more fine-grained
and time-limited manner of granting access. Set the `AZURE_ACCOUNT_NAME`
and `AZURE_ACCOUNT_SAS` environment variables to use a SAS token for
authentication. Note that if `AZURE_ACCOUNT_KEY` is set, it will take
precedence.

https://github.com/restic/restic/issues/2295
https://github.com/restic/restic/pull/3661
