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

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.

Inizia tracciando il percorso di creazione di model/Entity esposto tramite la CLI e il modo in cui le proprietà del modello vengono gestite durante la creazione e gli aggiornamenti delle risorse. Conferma la configurazione prevista per le varianti createdAt e updatedAt, quindi definisci test che dimostrino che la creazione imposta entrambi i timestamp e che le modifiche successive aggiornano solo il timestamp di modifica.