Support a sub-query that allows indirect reference between source objects

[fscott-ir]

For example, in CloudFormation you can have a VPCGatewayAttachment that associates a Gateway with a VPC. For a Gateway to be included in the threat model with a parent of the VPC, it would be necessary to map the parent of the gateway to the VPC via the Attachment:

    GatewayToInternet:
      Type: "AWS::EC2::VPCGatewayAttachment"
      Properties:
        VpcId:
          Ref: "VPC"
        InternetGatewayId:
          Ref: "InternetGateway"

One possible approach could be to leverage the $source action and ensure that values are carried through as needed.