JSON schema

General MR

Summary

Add JSON Schema file for BigBang umbrella values.yaml

Fixes #1469 (closed)

See Helm Docs - https://helm.sh/docs/topics/charts/#schema-files

This adds a file values.schema.json that is used to validate values passed in to the BigBang umbrella chart. For this first pass I added schema for the core packages (not including passthrough values), and the common fields (flux, git) for addons.