Key validation

[dhardy]

As noted here, it can be useful to validate that keys found in configuration files are known/accepted keys.

I haven't dived very far into how this crate works, but will try to lay out a strategy of how key validation could be supported:

• Add a LayerSort descriptor for each layer: default value, read-only file, env, override, validator, etc.
• In order to enable key validation, expect that default value layer(s) cover all supported keys
• The validator, as a layer / plug-in / function-of-the-Config, will enumerate all keys and report an error for any key from a layer other than default values which does not override a key from a default-value layer.

Hopefully this is clear enough? My primary reason for creating this issue is actually to ask whether there is interest in supporting this — it requires a little more complexity from layers (adding LayerSort, ability to query values from each layer).

[matthiasbeyer]

Hm. So key validation as you mention it is really a concern, yes. Right now, the idea would be to rely on the deserialization method for key verification, which would be serde most of the time... serde already supports denying unknown fields - would this suffice?