Types ambiguities

There are ambiguities in the types, comparing to both the official documentation and what we're actually getting on the endpoint. Should we match the former to align with the docs, or the latter to align with reality?

* `workerskv::WorkersKvBulkResult`: `unsuccessful_keys` does not seem to be optional. Rather, we just get an empty vector if the list is empty.
* `workerskv::write_bulk::KeyValuePair` doesn't reflect the official documentation, but neither does this one make sense either. Having everything optional isn't right. What should we do?
* `zone::Zone`: `plan` doesn't appear in the documentation, neither does it appear in the official TypeScript library.