[feature] Add cosign verification to helm chart artifacts stored in OCI repositories

[sillock1]

Is this Feature/Enhancement related to an Existing Problem? If so, please describe:

• Currently you can verify container images by adding certificate-oidc-issuer and certificate-identity annotations within the image manifest as completed here add keyless signature verification support #434 .
• You currently cannot do the same for helm charts that are signed using cosign and pushed to an OCI repository.

Describe Proposed Solution(s):

It would be nice to have similar verification enabled for OCI helm charts, something like this:

apiVersion: content.hauler.cattle.io/v1
kind: Charts
metadata:
  name: external-secrets-charts-example
spec:
  charts:
    - name: charts/external-secrets
      repoURL: oci://ghcr.io/external-secrets
      version: 0.18.2
      certificate-identity-regexp: ^https://github.com/external-secrets/external-secrets.*$
      certificate-oidc-issuer: https://token.actions.githubusercontent.com

Describe Possible Alternatives:

Currently the workaround is to put the helm charts within the image manifest like so:

apiVersion: content.hauler.cattle.io/v1
kind: Images
metadata:
  name: external-secrets-images-example
spec:
  images:
    - name: ghcr.io/external-secrets/charts/external-secrets:0.18.2
      certificate-identity-regexp: ^https://github.com/external-secrets/external-secrets.*$
      certificate-oidc-issuer: https://token.actions.githubusercontent.com