Add opt-in support for the experimental batcher helper -
open-telemetry/opentelemetry-collector#8122

When opting into this functionality the exporter's Consume*
methods will make synchronous bulk requests to Elasticsearch,
without additional batching/buffering in the exporter.

Co-authored-by: Carson Ip <[email protected]>

- Add flush timeout to sync bulk indexer, disable timeout_sender.
- Create a new backoff function for each call to Flush in the sync
  bulk indexer, since the function is stateful and not safe for
  concurrent calls to Flush.