Add Map ItemBatcher/ItemSelector

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The ItemBatcher provides a way to group large array inputs into larger batches for better performance

You can provide a MaxItemsPerBatch or MaxItemsPerBatchPath which limits the number of items in a batch, as well as a MaxInputBytesPerBatch and MaxInputBytesPerBatchPath which limits the size of the input in bytes.

The output will be a set of {"Items": []} payloads, one for each batch.

Batching Items

A Map State MAY have an "ItemBatcher" field, whose value MUST be a JSON object and is called the ItemBatcher Configuration. The ItemBatcher Configuration causes the interpreter to batch selected items into sub-arrays before passing them to each invocation. The interpreter will limit each sub-array to the maximum number of items specified by the "MaxItemsPerBatch" field, and to the maximum size in bytes specified by the "MaxInputBytesPerBatch" field.

The ItemBatcher Configuration MAY have a "BatchInput" field, whose value MUST be a Payload Template. An ItemBatcher Configuration MAY have a "MaxItemsPerBatch" field, whose value MUST be a positive integer. An ItemBatcher Configuration MAY have a "MaxInputBytesPerBatch" field, whose value MUST be a positive integer.

The default of "ItemBatcher" is the selected item. Put another way, the interpreter will not batch items if no "ItemBatcher" field is provided.

Both the "MaxItemsPerBatch" and "MaxInputBytesPerBatch" can be provided indirectly. A Map State may have "MaxItemsPerBatchPath" and "MaxInputBytesPerBatchPath" fields which MUST be Reference Paths which, when resolved, MUST select fields whose values are positive integers. A Map State MUST NOT include both "MaxItemsPerBatch" and "MaxItemsPerBatchPath" or both "MaxInputBytesPerBatch" and "MaxInputBytesPerBatchPath".

An ItemBatcher Configuration MUST contain at least one of "MaxItemsPerBatch", "MaxItemsPerBatchPath", "MaxInputBytesPerBatch", or "MaxInputBytesPerBatchPath".

https://docs.aws.amazon.com/step-functions/latest/dg/input-output-itembatcher.html
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