AWS::CloudFormation::StackSet - NewAttribute

## Scope
AWS::CloudFormation::StackSet supports only GetAtt of StackSetId. I would like to retrieve the names/IDs of the stacks within the StackSet as a new attribute.

## Expected behaviour
Return a list of stacks within the stackset. Ideally indexed in a useful manner (So you know which stacks are in which region)

## Test :
Can I create a stackset and then reference individual stacks within it later.
It would help to create a stack that creates a stackset that creates a sub-stack and creates an SNS-> Lambda ->SNS notification service so that failed updates to the sub-stack can alert someone by email. Currently there is no way for a stack to know it's own name to add suitable filters to the Lambda without making this a 2 step process. Create the stack and then create the alert. But with stacksets, you could do it all in a single template.

## Category
Management? Maybe Other? - Cloudformation

Start with the issue's description of AWS::CloudFormation::StackSet GetAtt and the existing StackSetId attribute. Clarify the returned stack names or IDs, how results should be associated with regions, and the expected behavior for referencing an individual stack; validate the design against the described nested StackSet and SNS-to-Lambda notification scenario.

Written by the indexing model from the issue text.