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

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## 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.

Start by tracing the model/Entity creation path exposed through the CLI and how model properties are handled during resource creation and updates. Confirm the intended configuration for createdAt and updatedAt variants, then define tests showing creation sets both timestamps and later modifications update only the modification timestamp.

Written by the indexing model from the issue text.