feat: extraEnvVars to support secret values

[riteshpradhan]

This allows importing values directly from secret.

Example:

---
extraEnvVars:
  - name: HTTPS_PROXY
    value: "https://example.com"
  - name: SECRET_VALUE
    valueFrom:
      secretKeyRef:
        name: my-secrets
        key: password

[CoolDuke]

This is actual the solution to read a client secret from a key vault (connected via CSI driver) while the other settings are defined as plain-text values. The current approach treating the whole config as a secret prevents this.

Would be great if someone could approve this PR and release a new helm chart version.