Allow execution of custom CloudFormation stacks

This would allow you to define custom resources for RDS, DynamoDB etc with regular CloudFormation and/or SAM support.

The alternative is that we have more "first class" resources, mapping them out in a way which is easier to configure than for example this [DynamoDB SAM](https://docs.aws.amazon.com/lambda/latest/dg/with-ddb-example-use-app-spec.html) example which is still quite verbose.