Add support for Rules

[jjanczyszyn]

Rules can be used to perform parameter validations based on the values of other parameters.

https://aws.amazon.com/blogs/mt/how-to-perform-cross-parameter-validation-using-aws-cloudformation-rules-and-assertions/

it seems that Rules aren't supported currently https://github.com/mapbox/accounts-airflow/blob/96be3b4f73d25c03fb7c77b24e311d0d1350b060/node_modules/@mapbox/cloudfriend/lib/merge.js#L30-L37

documentation on rules

cc @mapbox/platform