Add support for Password Flow for Customers in a Store

### Description
Hello team,

currently there seems to be no support for the [Password Flow for Customers in a Store](https://docs.commercetools.com/api/authorization#password-flow-for-customers-in-a-store) which requires hitting an endpoint not in included [here](https://github.com/commercetools/nodejs/blob/a6785130d36f3db35f62c9acde82e40fdff8e0f1/packages/sdk-auth/src/auth.js#L46):


Are there any plans on your side to support that flow OOTB? Or is it something you'd advise to handle using the [Custom Flow](https://commercetools.github.io/nodejs/sdk/api/sdkAuth.html#custom-flow)?

Inizia da packages/sdk-auth/src/auth.js, in particolare dall’elenco degli endpoint collegato nell’issue, e confrontalo con la documentazione commercetools Password Flow for Customers in a Store. Esamina come il Custom Flow esistente potrebbe gestire l’endpoint richiesto; il lavoro sarebbe completo quando saranno definiti un ambito di implementazione concordato e il comportamento del flow supportato.

Scritto dal modello di indicizzazione a partire dal testo della issue.