[sub-config] An ability to export a set of sub-settings as Qonfig::DataSet instance

For example:

```ruby
class Config < Qonfig::DataSet
  setting :database do
    setting :host
    setting :port
  end
end

config = Config.new
sub_config = config.sub_settings(:database) # => <Qonfig::DataSet:0x00007ff8>

sub_config.to_h
# =>
{ host: nil, port: nil }
```