Data that needs to be stored across service restarts or upgrades should be stored in a configurable data folder

The `data_folder` attribute in the `App.Config` should specify a (relative or absolute) folder where data that needs to persist across restarts or upgrades should be written
```
<LogsearchShipperGroup>
     <LogsearchShipper data_folder="data" ...
```

This includes NXLog data like the `nxlog.conf`, `CacheDir` and `SpoolDir`

If the folder specified doesn't exist it should be created