Enhancement: Add support for non-global Azure clouds

The `azure` backend previously only supported storages using the global domain
`core.windows.net`. This meant that backups to other domains such as Azure
China (`core.chinacloudapi.cn`) or Azure Germany (`core.cloudapi.de`) 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
