Implement linter support for CloudFormation

[gabenegron]

Description

As a user of seCureLI, I want the ability to configure a CloudFormation linter in my pre-commit hooks of my repo, so that I can ensure that my code base is scanned prior to any commits going through.

Technical Details

• Pre-Commit hooks are installed/configured after the language is detected as the primary language in the repo
• command: secureli init
• Linter to use: cfn-lint · PyPI

Acceptance Criteria

• Pre-commit hooks are configured and config is stored in the repo under /.secureli/config when a CloudFormation repo is detected.
• If there are any failing linting issues, per the list above, when a commit is attempted the commit is blocked and an error with details is displayed to the user.
• Readme & internal help is updated with supporting language info

[joseeliaschavez]

Info from Mikayla Cohen:

Secureli does not currently have any way to "specialize" JSON or YAML into a "CloudFormation" object. My advice is to leverage the hassattr() function (have it look for the 'AWSTemplateFormatVersion' field) to determine whether the parsed JSON contains a field unique to a Cloud Formation template. The caveat is that 'AWSTemplateFormatVersion' is an optional CloudFormation field, so if a user hasn't included it in their CloudFormation templates it won't detect the file as such. To work around this unavoidable limitation, my advice is to either/both (1) prompt the user with a question asking them if any of the JSON files discovered are 'CloudFormation' templates or (2) provide a Secureli configuration option that allows a user to 'force' install cfn-lint. There is one other method I was looking at, which was to create a custom type and use the typeof function to detect it, but I haven't had a chance to try it out. With this method, you would include all the fields in a CloudFormation object to give that object a type signature that could be used to identify it. This would, however, require more work than the previous method I described.

Info on the AWS CloudFormation linter, cfn-lint, can be referenced here: https://github.com/aws-cloudformation/cfn-lint