Support `insecureSkipTLSVerify` for private helm oci registries

[codering]

I use k8s to deploy docker-registry, and set up a self-signed certificate to host my helm charts, but fleet cannot currently obtain my charts. For example, my fleet.yaml

helm:
  releaseName: cert-manager
  chart: oci://docker-registry.xxxxx:5000/helm-charts/cert-manager
  version: v1.8.2
  values:
    installCRDs: true

the following error message will be reported:

.... level=fatal msg="Helm chart download: failed to do request: Head \"https://docker-registry.xxxxx:5000/v2/helm-charts/cert-manager/manifests/v1.8.2\": tls: failed to verify certificate: x509: certificate signed by unknown authority"