Fixes #20638: Document bulk create support in OpenAPI schema (#20777)

* Fixes #20638: Document bulk create support in OpenAPI schema

POST operations on NetBoxModelViewSet endpoints accept both single
objects and arrays, but the schema only documented single objects.
This prevented API client generators from producing correct code.

Add explicit bulk_create_enabled flag to NetBoxModelViewSet and
update schema generation to emit oneOf for these endpoints.

* Address PR feedback

- Removed brittle serializer marking mechanism in favor of direct checks
  on behavior.
- Attempted to introduce a bulk_create action and then route to it on
  POST in NetBoxRouter, but ran in to several obstacles including
  breaking HTTP status code reporting in the schema. Opted to simply

* Remove unused bulk_create_enabled attr

Co-authored-by: Jeremy Stretch <[email protected]>

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Co-authored-by: Jeremy Stretch <[email protected]>

Author: jnovinger