[Feature Request] Allow passing a post-authentication callable to OIDCAuth

The OIDCAuth class and group-based features added in 2.3.0 are great!

It would be useful for the OIDCAuth class to take a callable that is executed at the end of the OIDCAuth.callback method just before the redirect. This function would take the access token as single positional argument and have access to the session global, e.g.:

from dash import Dash
from dash_auth import OIDCAuth
from flask import session

def add_scopes_to_user(token):
    if scope := token.get("scope"):
        session["user"]["scopes"] = scope.split()

app = Dash(__name__)

auth = OIDCAuth(
    app,
    secret_key="aStaticSecretKey!",
    post_auth_callback=add_scopes_to_user,  # defaults to None
)

This feature would provide a lot of flexibility to app developers for different use cases:

• storage of additional fields from the token in the session (example above),
• add a user to the DB if not already existing,
• custom logging,
• etc.

I can submit a PR if this is a feature the maintainers would consider merging.

Inizia dal costruttore OIDCAuth e dal punto di ingresso OIDCAuth.callback descritti nella richiesta, quindi esamina il comportamento esistente di OIDCAuth relativo al reindirizzamento finale. Definisci l’argomento token del callable opzionale, l’accesso alla sessione e il momento dell’esecuzione, quindi aggiungi una copertura che dimostri che viene eseguito prima del reindirizzamento e ignorato quando non è impostato.