feat: export LearningPackage data in lp_dump command

[dwong2708]

Description

Fixes: #337

This PR updates the lp_dump management command to export real data from the LearningPackage model instead of using stub values.

Acceptance Criteria

• Give an error in the lp_dump command if the export process fails
• Export actual data from the LearningPackage model instead of stub values

Example of the content:

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[ormsbee]

Just some small requests.

[ormsbee]

For your follow-up PR, please add tests for this functionality (and the new stuff involving PublishableEntities).