Enhance kafka commit

Hello,

I think, it could be very useful to enhance "Kafka output" with a kind of timer option and associate it with the actual _kafka_coalesce_ parameter.

```
kafka_coalesce=100
kafka_flush_interval=5s
```

If one of theses conditions is met, commit the batch to Kafka.

Thanks

Start by locating the Kafka output implementation and the handling of kafka_coalesce; trace how batches are committed and where a flush interval could be integrated. Confirm the expected behavior for kafka_flush_interval=5s, then verify that a batch is committed when either the record-count or timer condition is met.

Written by the indexing model from the issue text.