Enh Req: Introduce Config metadata for troubleshoot of override/layered values

Hi, in many situations when multiple sources of configurations are used - it's beneficial to have ability of Config metadata to tell which value came from which source and became effective/actual at the end of sources processing.

Implementation of this could add extra feature, for example "config-meta" and once used/enabled - later have method like "get_meta" or "dump_meta".

So if let say I have something like such for config construction out of 2 sources:

fn make_config() -> Config {
  let current_env = env::var("ENV").unwrap();
  Config::builder()
      .add_source(File::new("env/service", FileFormat::Yaml))
      .add_source(File::new(("env/service-".to_string() + &current_env).as_str(), FileFormat::Yaml))
      .build()
      .unwrap()
}

and let say one of my YAMLs has such:

debug: false

and another:

debug: true

then that new method "dump_meta" could print something like this:

debug: false <= taken from service.yml
debug: true <= taken as override from service-dev.yml and effective

As I'm yet new to Rust - this is just proposal at this point from my side, which I could try to implement one day if no volunteers.

Start by reading Config::builder and the add_source processing flow described in the issue. Determine how layered values could retain their source information, then define and document a config-meta capability such as get_meta or dump_meta that reports each value's source and which value is effective after overrides.