Enhancement: Add config option to set Microsoft Blob Storage Access Tier

The `azure.access-tier` option can be passed to Restic (using `-o`) to
specify the access tier for Microsoft Blob Storage objects created by Restic.

The access tier is passed as-is to Microsoft Blob Storage, so it needs to be
understood by the API. The allowed values are `Hot`, `Cool`, or `Cold`.

If unspecified, the default is inferred from the default configured on the 
storage account.

You can mix access tiers in the same container, and the setting isn't
stored in the restic repository, so be sure to specify it with each
command that writes to Microsoft Blob Storage.

There is no official `Archive` storage support in restic, use this option at
your own risk. To restore any data, it is still necessary to manually warm up
the required data in the `Archive` tier.

https://github.com/restic/restic/issues/4521
https://github.com/restic/restic/pull/5046