fix: support force_enrollment in serializers used by bulk enrollment

[pwnage101]

ENT-8788

Testing

Before this PR (force_enrollment key was ignored from input):

$ python manage.py shell
>>> from enterprise.api.v1.serializers import EnterpriseCustomerBulkSubscriptionEnrollmentsSerializer
>>> serializer = EnterpriseCustomerBulkSubscriptionEnrollmentsSerializer(data={'enrollments_info': [{'email': '[email protected]', 'course_run_key': 'course-v1:edX+DemoX+Demo_Course', 'license_uuid': '5b77bdbade7b4fcb838f8111b68e18ae', 'force_enrollment': True}]})
>>> serializer.is_valid()
True
>>> serializer.validated_data.get('enrollments_info')[0].get('force_enrollment', False)
False

After this PR:

$ python manage.py shell
>>> from enterprise.api.v1.serializers import EnterpriseCustomerBulkSubscriptionEnrollmentsSerializer
>>> serializer = EnterpriseCustomerBulkSubscriptionEnrollmentsSerializer(data={'enrollments_info': [{'email': '[email protected]', 'course_run_key': 'course-v1:edX+DemoX+Demo_Course', 'license_uuid': '5b77bdbade7b4fcb838f8111b68e18ae', 'force_enrollment': True}]})
>>> serializer.is_valid()
True
>>> serializer.validated_data.get('enrollments_info')[0].get('force_enrollment', False)
True