Apply auto_set_fields when building entities in a transformer

[barnabasJ]

I mostly copied the code from here https://github.com/ash-project/spark/blob/66250ad02eb74b309171795499de7c22fe9c44e9/lib/spark/dsl/entity.ex#L80

Contributor checklist

• [x] Bug fixes include regression tests
• [ ] Chores
• [ ] Documentation changes
• [ ] Features include unit/acceptance tests
• [ ] Refactoring
• [ ] Update dependencies

[zachdaniel]

🚀 Thank you for your contribution! 🚀