django-mfa2 User_Keys uses project DEFAULT_AUTO_FIELD instead of its integer migration PK

[BVengo]

mfa.models.User_Keys does not declare its primary key and the mfa app config does not set default_auto_field.

This means the host project's DEFAULT_AUTO_FIELD is used at runtime, despite django-mfa2s migrations
relying upon AutoField. This causes a runtime/schema mismatch in projects that use a non-integer default key (e.g. UUIDs).

To fix, just specify the default_auto_field in apps.py:

from django.apps import AppConfig


class myAppNameConfig(AppConfig):
    name = "mfa"
    verbose_name = "A Much Better Name"
    default_auto_field = "django.db.models.AutoField"

I'll open up a PR for this. We're trying to implement django-mfa2 in a project at work, and we had to specify a custom app config to workaround it in the interim.