Add spec file so that cfn-lint understands custom Rain directives

[iainelder]

I want to use Rain's special directives such as !Rain::S3Http in my templates, and I also want to lint my templates with cfn-lint.

Today if I use that directive, cfn-lint gives an error like this:E3012 {'Fn::Rain::S3Http': 'substack.yml'} is not of type 'string'..

cfn-lint support custom spec files that describe extensions to the core CloudFormation elements.

It would be great if the rain repo included a custom spec that allows cfn-lint to accept its custom directives.

That's what aws-sso-util does: https://github.com/benkehoe/aws-sso-util/blob/0f9470828b60cbca643b997dafc37f1f0d087e14/macro/cfn-lint-spec.json

See original issue in the cfn-lint repo for conext: aws-cloudformation/cfn-lint#4006