Enforce validation of the configuration

[xaltsc]

I made the mistake of setting upstreams = [ { publicKey = "..."; ...} ...]; instead of public_key. Of course, this does not generate any TOML error, but it doesn't add the keys to nix.settings.

It would be nice if upstreams could be a listOf of submodules with defined attributes, so that such an error is catched at build time; especially since this does not conform to the usual camelCase nix naming of things.

[NotAShelf]

This is somewhat trivially doable. Sorry for the radio silence, I've got some work staged that's holding me back at the moment but I'll implement config validation shortly for the next release.