Multi-Stack support

[alynchos]

The supported cdk functions are:

        {
            templateName: 'Ref', resolver: () => 'aws'
        },
        {
            templateName: 'Fn::GetAtt', resolver: fnGetAtt
        },
        {
            templateName: 'Fn::Join', resolver: fnJoin
        }

In a multi-stack application, "Fn::ImportValue" is used to reference values from other stacks. If all stacks are locally synthesized, aws-cdk-state-machine-asl should grab that value for you.