Enhancement: support connection to rest-server using unix socket

Restic now supports connecting to rest-server using a unix socket for
rest-server version 0.13.0 or later.

This allows running restic as follows:

```
rest-server --listen unix:/tmp/rest.socket --data /path/to/data &
restic -r rest:http+unix:///tmp/rest.socket:/my_backup_repo/ [...]
```

https://github.com/restic/restic/issues/4287
https://github.com/restic/restic/pull/4655
