amplify-appsync-simulator doesn't support DynamoDB Batch operations

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no. it's about the aws-amplify cli.

**Is your feature request related to a problem? Please describe.**
amplify-appsync-simulator with DynamoDB datasource doesn't support Batch operations like BatchGetItem,BatchDeleteItem, BatchWriteItem.
so I can't test it locally.

**Describe the solution you'd like**
simply support them

**Describe alternatives you've considered**
sorry I have not considered alternatives.
it seems straightforward to me, if any risks please let me know.

**Additional context**
I'll submit my PRs.

it seems working just fine in my project.
I want know how to run tests regarding amplify-appsync-simulator with dynamodb datasource, other than install it into existing projects and run queries actually.
(it doesn't seem the package has tests for data-loaders)