Add official Github action for openapi-diff

Openapi-diff is well suited to be integrate into Github actions in order to:

• Highlight breaking in a pull request
• Document changes in OpenAPI
• etc.

For example, today we have an internal github repo which on every commit triggers generating Redoc OpenAPI documentation and publishes it to Github pages for the repository. In this documentation we'd like to include a openapi-diff markdown report of the changes.

Or, when ever we do a pull request we want to review the changes made. Openapi-diff will allow us to identify automatically breaking changes.

There are a couple of unofficial limited Github actions based on other openapi diff tool

• https://github.com/marketplace/actions/openapi-review
• https://github.com/marketplace/actions/openapi-diff

Inizia esaminando gli entry point esistenti del repository e la configurazione di release o automazione; l’issue non indica file né test. Confronta il comportamento richiesto con le action non ufficiali collegate e determina come un’action ufficiale dovrebbe esporre i risultati di OpenAPI diff e i controlli delle breaking changes. Il lavoro è completato quando una GitHub Action utilizzabile è documentata e validata per i workflow di pull request e di generazione dei report.