Hoisting fields for BatchCreate* requests blocked by linter

The [docs for batch creation](https://google.aip.dev/233#request-message) state:
Other fields may be "hoisted" from the standard Create request, which means that the field can be set at either the batch level or child request level.

However, this gets flagged by the linter: https://linter.aip.dev/233/request-unknown-fields