Make deny_unknown_fields a runtime feature

`settings_deny_unknown_fields_by_default` is a useful feature (that's why its enabled by default), but in some applications it is desirable to have the _option_ to be strict. For example, be strict when validating new configs but permissive when parsing existing files from disk.

We should find a way to support such use cases. Maybe we can track which fields were not deserialized in the `Deserializer` impl?