Support auto-updated model properties "createdAt" and "updateAt"

まだ誰も着手していません。

## Feature proposal
While creating a model/Entity from cli, I have an option to create a date field. A common example would be a user model with created_at and updated_at fields (or any variants of createdAt and updatedAt). Typically the behaviour is that on new resource creation, I would like to set the created_at and updated_at as the current time. on subsequent updates, I would like to update the updated_at timestamp with the time of operation.

Given that created_at needs to be populated at creation and updated_at needs to be updated with every modification, it can be a nice inbuilt feature to have with models where I can say that a particular field acts as created_at and another one as updated_at and loopback can take care of updating these fields, saving me some lines of code.

まず、CLI から公開されている model/Entity の作成パスと、リソースの作成および更新時にモデルのプロパティがどのように処理されるかを追跡します。createdAt と updatedAt の各バリアントに対する意図した設定を確認し、作成時に両方のタイムスタンプが設定され、その後の変更では変更用タイムスタンプのみが更新されることを示すテストを定義します。

索引モデルが issue の本文から書いたものです。