stepfunctions: Add support for ItemsPointer for DistributedMap's items reader

### Describe the feature

It is no longer necessary to S3 item to be an JSON array of items which should be processed. Now it is possible to specify path in the S3 file which points to a JSON array which contains items which should be iterated.

https://docs.aws.amazon.com/step-functions/latest/dg/input-output-itemreader.html#itemsource-example-json-data shows `ItemsPointer` which adds support for this.

### Use Case

This feature is supported by AWS, I would like to use it in my project

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS CDK Library version (aws-cdk-lib)

-

### AWS CDK CLI version

-

### Environment details (OS name and version, etc.)

-

Start with the AWS Step Functions ItemReader documentation linked in the issue, especially the ItemsPointer example, then locate the TypeScript constructs for DistributedMap and its items reader. Confirm how the existing reader models S3 JSON input; done means exposing ItemsPointer and covering the resulting synthesized configuration with tests.

Written by the indexing model from the issue text.