Make entity data retrieval extensible

[dwong2708]

Make entity data retrieval extensible

Once we have the first version of the lp_dump file, we plan to retrieve the data via interfaces instead of relying on subclasses of PublishableEntityMixin and PublishableEntityVersionMixin.

Acceptance Criteria

• Retrieve entity data for the TOML files using the PublishableEntity interface.
• Retrieve entity version data for building the TOML files using the PublishableEntityVersion interface.