[Task] OpenAPI - Describe the CRON and WebHook endpoints

[taorepoara]

What should be done

Our internal API (callable only by the Lenra apps) contains CRON and WebHook management endpoints that are not described yet.

Describe those endpoints that are defined in the server repo: https://github.com/lenra-io/server/blob/beta/libs/application_runner/lib/router.ex#L29

Decribe them in the OpenAPI dedicated to the internal API: src/internal-api.yml

Technical recommandation

Define common structures (that could be used in many requests and responses directly in the file, in the components.schemas definition (that does not exist yet).