GDPR compliance fix: Support parameter to auditlog.register to change how auditlog populates object_repr

[shundread]

We have a custom User model that includes user email / name into the string representation:

    def __str__(self):
        if self.first_name and self.last_name:
            return f"{self.last_name} {self.first_name} ({self.email})"
        else:
            return self.email

Due to django-auditlog's grabbing instances' string representation for its object_repr, this isn't enough to reduce the amount of PII we're writing onto our logs:

 auditlog.register(
    User,
+    mask_fields={"first_name", "last_name", "email"},
 )

We are putting in a workaround with signals at the time being, but it would be nice if similar projects can eliminate the same kind of limitation through a feature in the official package, using something like:

 auditlog.register(
    User,
+    mask_fields={"first_name", "last_name", "email"},
+    object_repr_field="pk",
 )

For reference, our workaround looks like this:

@receiver(post_log, sender=User)
def anonymize_user_object_repr(sender, instance, log_entry, log_created, **kwargs):
    if log_created and log_entry and log_entry.object_repr:
        log_entry.object_repr = instance.pk
        log_entry.save(update_fields=["object_repr"])

I have skimmed the classes involved on django-auditlog and it looks to me like some small changes in LogEntryManager and AuditLogModelRegistry could be sufficient to support the feature and spare the need for a workaround for our project and potentially others.

I'm happy to provide a pull request if this feature seems sensible.

[shundread]

Being unable to affect how object_repr may affect users of Django's default User model, as its __str__ method resolves to username, and in many services username will be the user's email.