Allow to get the username by email

[M123-dev]

Problem description

In the mobile-app we display something like "Hello Teolemon". (Not as a one time dialog, at the top of the settings).

This doesn't work when a user uses their email to login.

Desired solution

A simple endpoint to recieve the username.

Alternatives you have considered

A structured answer from a login endpoint, but thats out of the scope of this what seems to me very simple issue.

Additional context

#1833

Number of products impacted

1, the official open food facts mobile app. 😄