Added support for working with custom Django User Model.
The following DRF_STRIPE settings can be used to customize how Django creates User instance using Stripe Customer
attributes (default values shown):
python
DRF_STRIPE = {
"DJANGO_USER_EMAIL_FIELD": "email",
"USER_CREATE_DEFAULTS_ATTRIBUTE_MAP": {"username": "email"},
}
The `DJANGO_USER_EMAIL_FIELD` specifies name of the Django User attribute to be used to store Stripe Customer email. It
will be used to look up existing Django User using Stripe Customer email.
The `USER_CREATE_DEFAULTS_ATTRIBUTE_MAP` maps the name of Django User attribute to name of corresponding Stripe Customer
attribute, and is used during the automated Django User instance creation.