Support preloading of associations

Consider a model with references inside embedded collection:

```ruby
class Example
  include ActiveData::Model

  embeds_many :users do
    references_one :user
  end
end
```

accessing `users` causes 1 model query per each element of the `users` collection.

### Suggestion

Add the `preload` method to eagerly preload selected associations to avoid N+1's queries.