feat: expose create_by field in PublishableEntityMixin [FC-0062]

[rpenido]

Description

This PR exposes the create_by field in PublishableEntityMixin to facilitate it's use.

More Information

• https://github.com/openedx/edx-platform/pull/35476#discussion_r1757815110

Part of:

• https://github.com/openedx/frontend-app-course-authoring/issues/1190

Testing Instructions

Check https://github.com/openedx/frontend-app-course-authoring/pull/1275

---

Private ref: FAL-3802

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