changefeedccl: add more reliable benchmarks

make some unit-test-level benchmarks that are more reliable and understandable

- one that runs a changefeed but replaces the rangefeed rpc with a stream of data, so we can produce load without involving the whole database. use a null sink
- a benchmark for the kafka v2 sink alone, using kfake from franz-go to avoid having to spin up a kafka

add them to roachperf

once these exist and are working, we can track them over time using roachperf &/ its successor

Jira issue: CRDB-48330