BTN-2255 - Remove Generated Models

Removes generated/ and schemas/

Toward a holistic, fully integrated PDM with only wholesome organic Pydantic models and no filler ingredients. ☸

• All the auto-generated model classes have been removed.
• Adds entities.proposal and entities.shared
• Breaking change: Replaced proposal.targets with common.target
• The changes deleting schemas/ were already reviewed as part of !91 (merged)

Very happy to get feedback on naming / organisation of the entities.

My suggestion when reviewing this would be to tick the viewed box for all the deleted files and basically ignore them. Everything interesting is in entities/ now and that's what (IMHO) is worth looking at - and the tests.