cdc: make format=parquet work with changefeed.event_consumer_workers

We should do some benchmarks comparing `format=parquet` to `format=json` to see if there is room for improvement in terms of throughput. If so, we should remove the limitation where `event_consumer_workers` is incompatible with `format=parquet`.

Jira issue: CRDB-29635

Epic CRDB-37096

Start by locating the changefeed implementation for format=parquet and the event_consumer_workers setting. Benchmark parquet against json for throughput, then determine whether the results justify removing their incompatibility. Done means the benchmark supports the change and parquet works with event_consumer_workers without the current limitation.

Written by the indexing model from the issue text.