Support `additional_attributes`

Some thoughts about `additional_attributes`: usually, users will want it to be required from the perspective of the DB. However, they might want it to bypass the request validation since the user can omit it.

One way around this would be to add another prop to override the requirement that `nullable: false` fields must appear in the request.

No file, test, or entry point is named. Start by tracing how `additional_attributes`, `nullable: false`, and request validation are handled, then clarify whether the change should affect database requirements, request requirements, or both. Done should include a defined behavior for omitted fields and coverage for the relevant validation cases.

Written by the indexing model from the issue text.