**Reason for this change**
- For `DistributedMap` state of StepFunctions, `IItemReader` currently only allows S3 bucket as input source to be declared statically in CDK.
- In other words, current CDK implementation only caters to static use-case where we know either `bucket` or `bucketName` (from which we can create `IBucket`) and pass it to `IItemReader`.
- Whereas via AWS management console, if we create `DistributedMap` manually, then we can also convey S3 source dynamically using State Input / JsonPath.
- In other words, for dynamic use-case, we will neither have `bucket` nor `bucketName` i.e. we only know state input variable which will convey `bucketName` e.g. `$.bucketName`.
- So, if we want to use `IItemReader` for dynamic use case also, then, to avoid making breaking change (e.g. changing type of `bucket` from `IBucket` to `string`), we will:
  - (1) need to make `bucket: IBucket` an optional prop in `ItemReaderProps` (refer [Making properties optional](https://github.com/aws/jsii/blob/main/packages/jsii-diff/BREAKING_CHANGES.md#making-properties-optional))
  - (2) and add another optional field `bucketNamePath: string` to convey state input variable name (e.g. $.bucketName)

**What changes are being made?**
- Add `bucketNamePath` as optional prop in `ItemReaderProps`.
- Make `bucket` an optional prop instead of required prop in `ItemReaderProps`.
- Adapt implementing classes of `IItemReader` to handle `bucket` being optional (refer [Making properties optional](https://github.com/aws/jsii/blob/main/packages/jsii-diff/BREAKING_CHANGES.md#making-properties-optional)).
- Add `validateItemReader` in `IItemReader` which implementing classes will implement to handle mutual exclusivity of `bucket` and `bucketNamePath`.
- Modify `DistributedMap:validateState` to validate `IItemReader` if present.
- Modify README to explain and add examples relevant to changes.
- Add new unit-tests for `DistributedMap`.
- Add new integration tests to confirm vali

**How are these changes tested?**
- Build changes: `cd ./packages/aws-cdk-lib/ && yarn build aws-stepfunctions && yarn watch`
- Add and run new unit-tests for `DistributedMap`: `yarn test aws-stepfunctions/test`
- Run `yarn rossetta:extract -d aws-stepfunctions -v` to confirm validity of README changes
- Add new integration test and run (with snapshot created):
  - Build once and watch: `npx lerna run build --scope=@aws-cdk-testing/framework-integ && yarn watch`
  - Test: `yarn build --directory test/aws-stepfunctions/test && yarn integ test/aws-stepfunctions/test/integ.item-reader-path-s3-object.js --update-on-failed`
  - Verified expected step function execution result during snapshot creation

- for IItemReader, if bucket is not given, then policy statement can allow step function to list any bucket.