arrow

PARQUET-1993: [C++] expose way to wait for I/O to complete

For applications that want to pipeline I/O and Parquet decoding, and don't want to be blocked on I/O (instead decoding data as it becomes available), this allows them to pre-buffer the data they're about to read, then explicitly be signaled when the buffering completes.

Once we also make the Parquet reader re-entrant, we can expose this under a more ergonomic API as `Future<AsyncGenerator<shared_ptr<RecordBatch>>> parquet::arrow::FileReader::ReadRowGroupsAsync(...)`

Closes #9613 from lidavidm/parquet-1993

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>