Enhancement: Add support for non-global Azure clouds

Restic backups on azure only worked for storages on the global domain `core.windows.net`. This meant that backups to other domains such as Azure China (`core.chinacloudapi.cn') were not supported. Restic now allows overriding the global domain using the environment variable `AZURE_ENDPOINT_SUFFIX'.

https://github.com/restic/restic/issues/2468
https://github.com/restic/restic/pull/4387