✨(backend) support Authorization code flow

Integrate 'mozilla-django-oidc' dependency, to support
Authorization Code flow, which is required by Agent Connect.

Thus, we provide a secure back channel OIDC flow, and return
to the client only a session cookie.

Done:
- Replace JWT authentication by Session based authentication in DRF
- Update Django settings to make OIDC configurations easily editable
- Add 'mozilla-django-oidc' routes to our router
- Implement a custom Django Authentication class to adapt
'mozilla-django-oidc' to our needs

'mozilla-django-oidc' routes added are:
- /authenticate
- /callback (the redirect_uri called back by the Idp)
- /logout

src/backend/core/api/utils.py

@@ -1,14 +0,0 @@
-"""
-Utils that can be useful throughout the People core app
-"""
-from rest_framework_simplejwt.tokens import RefreshToken
-
-
-def get_tokens_for_user(user):
-    """Get JWT tokens for user authentication."""
-    refresh = RefreshToken.for_user(user)
-
-    return {
-        "refresh": str(refresh),
-        "access": str(refresh.access_token),
-    }