Allow SAS tokens to be used for Azure Blob backend

[waza-ari]

Summary

As a security-aware user,
I want to be able to use Azure SAS tokens
So that I can limit the required access to the storage account to a minimum

Context

Restic does support multiple ways of authenticating against Azure, two of which are likely to be relevant for K8up: AZURE_ACCOUNT_KEY or AZURE_ACCOUNT_SAS.

Using a SAS token is preferable, as it provides restricted access (scope wise and permission wise) to parts of the storage account (e.g. only the container used for backup), instead of providing all access to the entire storage account. Currently, the AzureSpec only exposes the accountKeySecretRef, it would be nice if something similar could be done for the SAS token.

Out of Scope

No response

Further links

https://restic.readthedocs.io/en/latest/030_preparing_a_new_repo.html#microsoft-azure-blob-storage

Acceptance Criteria

No response

Implementation Ideas

No response