List of classes

The current documentation sates lists that a list may include only:

- Hash
- Array
- String
- Numeric
- Boolean
- IO
- Set
- nil

I've been looking over the code and was hoping theres was a way to define a formal class within a list. For example:

```ruby
class Order
include Aws::Record
list_attribute :line_items, of: LineItem
end
```

The exact interface for how this would be accomplished is up for debate, but if this is a feature within the scope of this gem I would be willing to contribute it.