feat(helm-chart): Add ability to deploy extra manifests

[mkilchhofer]

Add ability to deploy extra manifests within the helm chart

It is common now that helm charts offer the ability to deploy custom resources along the application. It is especially useful for managing Secrets (eg. via external-secrets.io / Secrets Store CSI Driver) or deploying more granular NetworkPolicy (eg. CiliumNetworkPolicy).

Charts in the community that supports this:

• all Argoproj charts (Argo CD, Argo workflows, ...)
• grafana charts (Grafana, Promtail, Loki, etc.)
• most Bitnami charts
• VictoriaMetrics charts
• prometheus-community charts

How to use

See example in values.yaml:

extraObjects:
  - apiVersion: external-secrets.io/v1beta1
    kind: ExternalSecret
    metadata:
      name: my-external-secret
      namespace: '{{ .Release.Namespace }}'
    spec:
      secretStoreRef:
        kind: ClusterSecretStore
        name: my-secret-store
      target:
        name: my-kubernetes-secret
      data:
        - secretKey: secretKey
          remoteRef:
            key: /path/to/my-secret

Testing done

[Describe the testing you have done before submitting this PR. Please include both the commands you issued as well as the output you got.]

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.

[joaquimrocha]

@tormath1 , can you take a look?

[pothos]

Sorry for the review delay, we'll try to assign someone in tomorrow's planning call flatcar/Flatcar#1103

[tormath1]

LGTM - seen this on other projects. Thanks for the addition!