Add setting to specify user model manager name.

[ghost]

Problem

Have custom manager set on User model which filters out certain types of users (sometimes this is desirable). There is a second, all_objects manager which is the default one and doesn't filter anything. rest_framework_simplejwt.authentication.JWTAuthentication.get_user() is hard-coded to use User.objects.

Solution

Add api_settings option to configure this, default being objects.