[otap-dataflow] parquet exporter flush open writers after timeout

[albertlockett]

After what was implemented in #488, the parquet exporter only flushes files if:

• a) the files surpass some number of rows
• b) the exporter receives a shutdown message

If neither of these things ever happens, the open writer will never flush the file. This means that data may not become visible for quite a long time after it was received. We should probably have some capability to periodically flush writers in order to prevent this.

There are multiple ways to go about this timing and we can consider the best approach or even using a combination. For example, from the perspective of some unflushed file, we could flush on a timeout computed from either:

• Time since first batch
• Time since last batch

The batch processor is currently using time since last batch https://github.com/open-telemetry/otel-arrow/pull/347/files

This should be implemented after prerequisites:

• [otap-dataflow] emit control message from pipeline components using effect handler #498
• [otap-dataflow] pipeline engine timer control #497