Add 'comments' field to schema
https://repo1.dso.mil/big-bang/bigbang/-/blob/master/docs/assets/configs/example/keycloak-dev-values.yaml and https://repo1.dso.mil/big-bang/bigbang/-/blob/master/docs/assets/configs/example/keycloak-prod-values.yaml have a comments
key at the top that is not represented in the chart's schema.
This MR adds it so that those config examples are valid.