Add helm values schema

[dan-fernandes]

As a programmer I would like a schema for values.yaml to know what values are available, and to validate a given values file for correctness.

Acceptance Criteria

1. A helm values schema exists
2. The schema is referenced in the values.yaml for the yaml language server
3. The values file is tested against the schema as part of pre-commit / CI (e.g. with helm template)