* feat(user_api_key_auth.py): (enterprise) allow user to enable custom auth + litellm api key auth
makes it easy to migrate to proxy
* fix(proxy/_types.py): allow setting 'spend' for new customer
* fix(customer_endpoints.py): fix updating max budget on `/customer/update`
Fixes https://github.com/BerriAI/litellm/issues/6920
* test(test_customer_endpoints.py): add unit tests for customer update endpoint
* fix: fix linting error
* fix(custom_auth_auto.py): fix ruff check
* fix(customer_endpoints.py): fix documentation