fix(authentication-local): Local Auth - Nested username & Password fields

[eXigentCoder]

In my application, I store all the authentication related information related to the user in a nested object. This nested subobject includes for example, the last login time, the password hash & salt, last login IP address etc and in my case it is called _auth. The problem is that currently the library only allows you to confirgure the username and password fields for authentication to only be on the top level of the entity.
A test has been added for the nested field case and lint rules passing locally.

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[eXigentCoder]

Hey team, just wanted to check if you need anything further from me to get this merged in?

[daffl]

Apologies for the delay and thank you for the thorough pull request!