fix: add missing author value when restoring entities

[dwong2708]

Resolves: #446

Updates:

• Removed the created_by serializer field because it is no longer necessary for serialization, although still required for entity/collection creation.
• Updated logic so created_by reflects the user executing the restore operation rather than the user who created the backup.
• Added unit tests to validate the updated behavior.

Result:

[openedx-webhooks]

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