It looks like sidecars are only used for transferring row data with `WriteRequestPB` and `ReadRequestPB` message responses. Example found [here](https://github.com/yugabyte/yugabyte-db/blob/8f0a8c2282b5cec16a2e09a8308a192d69fb9e30/src/yb/tserver/tablet_service.cc#L475).
