Convert CDK VPC subnet list into CFN subnet list type

example.ts

var subnetIds:String[] = [];
vpc.privateSubnets.forEach((value: ISubnet)=>{
  subnetIds.push(value.subnetId)
});

const template = new cfninc.CfnInclude(this, "Template", {
  templateFile: 'template.yaml',
  parameters: {
    "ClusterName": cluster.clusterName,
    "Subnets": cdk.Fn.split(",", subnetIds.join(",")),
    LogGroupName: logGroup.logGroupName
  }
});