[Feature] Add option to select batch construction strategy

[Devingryu (devingryu)]

Adds the batchStrategy option to make batch construction behavior explicit. (See also: #266)

• Adds batchStrategy option to ParallelConsumerOptions:
  • SEQUENTIAL: keeps the existing strict ordered behavior; in KEY/PARTITION modes only one record is taken per shard in a scheduling cycle, so shard-level concurrency remains effectively one-at-a-time. (Default)
  • BATCH_MULTIPLEX: allows taking multiple records from the same shard in one scheduling cycle, while still allowing a batch to contain records from multiple shards.
  • BATCH_BY_SHARD: allows taking multiple records from the same shard and additionally ensures that each batch contains records from only one shard.

Overview of the implementation

• The work-selection path in ProcessingShard was changed so that, in ordered modes, the shard scan no longer always stops after the first taken record.
• Previously, the loop effectively behaved as strictly sequential because it would stop scanning the shard immediately after a take.
• With BATCH_MULTIPLEX and BATCH_BY_SHARD, the loop now continues gathering records from the same shard until batchSize is reached; SEQUENTIAL still takes the old stop-after-one path.
• On the batch-construction side, BATCH_BY_SHARD uses shard-aware partitioning, so each batch is limited by batchSize and also split when the shard changes.

Concerns

1. Semantics in UNORDERED
UNORDERED + BATCH_MULTIPLEX currently behaves the same as size-based batching, while UNORDERED + BATCH_BY_SHARD means one topic-partition per batch. This is now documented and tested, but it would be good to confirm that this is the intended public API behavior.

2. Ordered work selection vs. final batch construction
   The change splits behavior into two stages: how many records may be taken from a shard, and how the taken records are grouped into final batches. Please review whether this separation matches the intended model for the option.

Checklist

• Documentation (if applicable)
• Changelog

P.S. Since this repository does not appear to be actively maintained right now, I would particularly appreciate feedback from someone with a strong understanding of the project's internals.