Attach key to type error generated from Config::get_<type>()

[yuja]

This commit extracts low-level get_value() to preserve the origin as much as possible. get::() would run Value-to-Value deserialization, and the origin field would be lost there.

For scalar types, we could instead leverage get::() as the deserializer implemented for Value type invokes Value::into_() function:

fn get_string(&self, key: &str) -> Result<String> { self.get(key) }

[yuja]

Sure, rebased. The tests passed locally.

[matthiasbeyer]

Thanks for contributing! :tada: