#241 First pass at CloudFormation parameter validation

[corey-cole]

Issue #, if available: 241

Description of changes:
Rules for checking CloudFormation parameter overrides. See AWS CLIv2 docs for more details.

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[satyakigh]

The ARN regex rejects valid partitions and services containing -: \w+ does not match aws-cn, aws-us-gov, or services such as execute-api. The selector still includes these values because /^arn:aws/ matches their prefix, so they are reported malformed. I reproduced both arn:aws-cn:s3:... and arn:aws:execute-api:... exiting 19; using [\w-]+ for partition and service makes both pass. Please update the regex and add representative tests.