Store the locations of loaded config files for debugging

[Kixunil]

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It may be useful to be able to debug which config files were loaded. An example where this might've been useful: https://github.com/rootzoll/raspiblitz/issues/3447 I've seen some services doing such logging (Tor, I think) and it was handy.

The proposed API is to have a Metadata struct returned together with the configuration. It will also contain program path and spans. And potentially more things in the future. We can simply use Vec<PathBuf>. It will slow down startup a bit but hopefully not much. If this ever becomes a problem we may offer a feature/cfg option to not fill it (empty is zero cost).