OPAL should have JSON-Schema validator capability

[avo-sepp]

Describe the solution you'd like A bad OPAL fetch can result in unexpected and hard to troubleshoot errors. Especially if the maintainers are not as familiar with the original system. (Interns, New Hires, etc.) I would like to be able to supply a JSON-Schema along with any fetcher configs. OPAL will validate the output of process() from whatever fetcher got the data against the JSON-Schema. If the validation fails, OPAL should report "unhealthy" on its health check endpoint. Preventing Kubernetes rollouts from continuing a rollout.