[Helm] allow `enabled` as key in values schema

[wrdls]

When using Helm and subcharts, a common pattern is to add a condition to the subchart(s) to enable/disable them.

See https://helm.sh/docs/chart_best_practices/dependencies/#conditions-and-tags

Example Chart.yaml:

apiVersion: v2
name: cert-manager
version: 1.2.3
dependencies:
  - name: cert-manager
    version: v1.19.1
    repository: https://charts.jetstack.io
    condition: cert-manager.enabled
  - name: cert-manager-google-cas-issuer
    version: 0.10.1
    repository: https://charts.jetstack.io
    condition: cert-manager-google-cas-issuer.enabled

Example values.yaml:

cert-manager:
  enabled: true

cert-manager-google-cas-issuer:
  enabled: false  # <-- key not allowed

The above fails with

Error: values don't meet the specifications of the schema(s) in the following chart(s): cert-manager-google-cas-issuer: - (root): Additional property enabled is not allowed

In the cert-manager chart this is allowed: https://github.com/cert-manager/cert-manager/blob/master/deploy/charts/cert-manager/values.schema.json#L54-L56