AWS::DynamoDB::Table GetAtt for DynamoDB Stream Name

### Name of the resource

AWS::DynamoDB::Table

### Description

For `AWS::DynamoDB::Table`, CloudFormation [supports](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-dynamodb-table.html#aws-resource-dynamodb-table-return-values) `!GetAtt` on `StreamArn` to return the ARN of a DynamoDB Stream when `StreamSpecification` is configured.

There are cases where only the Stream's name is desirable. For example, SAM's [DynamoDBStreamReadPolicy](https://github.com/aws/serverless-application-model/issues/289). The name can be extracted from the ARN: given DynamoDB Table `MyDynamoDbTable`, we get the name like so:

```yaml
!Select [3, !Split [/, !GetAtt MyDynamoDbTable.StreamArn]]
```

Whilst the above is a sufficient workaround for now, may `GetAtt` be extended to support name retrieval in addition to ARN?

```yaml
!GetAtt MyDynamoDbTable.StreamName
```

Start with the CloudFormation AWS::DynamoDB::Table return-values documentation and the linked SAM DynamoDBStreamReadPolicy issue. Confirm the requested StreamName GetAtt behavior and identify whether this roadmap repository names an implementation or validation path; done means the requested attribute is supported and documented.

Written by the indexing model from the issue text.