Add JSON output format to 'pdk validate'

[natemccurdy]

Use Case

I use pdk in git hooks and CI systems to validate Puppet changes. Right now, I have to do string parsing of the stdout of pdk validate or output in junit XML. String parsing is not sustainable, and XML is... well, XML.

JSON is supported by all of the native tooling and languages I use, and parsing it is very easy.

Describe the Solution You Would Like

pdk validate --format=json

This command should output the results of pdk validate to STDOUT as a JSON blob.
Ideally, there'd be a JSON spec that the output conforms to.

The JSON blob should contain information you'd expect to see in a code review. The exact schema is TBD, but at a minimum, these come to mind:

• Path of file that failed validation
• Line number of failure
• Character (column) number of the failure (if available)
• The name of the linter error
• A description or message from the linter for that failure.
• Severity of the failure.
• The type of validation or name of the linter that was run.

Describe Alternatives You've Considered

Tried string parsing the stdout, but this is entirely unmaintainable as programs update and stdout evolves.

Tried Junit XML, but this is a very specific format of XML designed for the Java world. Using it feels like stepping back in time, and not in a good way.