[codegen] Add openAPI attribute to validation and recommendation

[sebastien-rosset]

• Add "OpenAPI" attribute such that validators can access the OpenAPI specification. For example, this is useful to determine the version of the OpenAPI spec, but this could also be useful potentially to perform cross attribute validation.
  For example, I would like to be able to do something like this: https://github.com/CiscoM31/openapi-generator/pull/16/files#diff-74008c7d7a3f7e768ea177ffd70c334dR91

• Add code comments to existing method.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

[jimschubert]

This looks good. I'm looking forward to the 3.1 validation!