Enhancement: Add support for Microsoft Blob Storage access tiers

The new `-o azure.access-tier=<tier>` option allows specifying the access tier
(`Hot`, `Cool` or `Cold`) for objects created in Microsoft Blob Storage. If
unspecified, the storage account's default tier is used.

There is no official `Archive` storage support in restic, use this option at
your own risk. To restore any data, it is 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
