feat(helm): sign helm chart using helm-sigstore plugin

[neilime]

More infos about signing helm charts :

1. Generate a .prov file using helm sign command : https://helm.sh/docs/topics/provenance/
2. Upload the .prov file using helm sigstore upload <path_to_packaged_chart>
3. Add following infos in Chart.yaml

annotations:
  artifacthub.io/signKey: |
    fingerprint: <empreinte de votre clé>
    url: <url publique de la clé ou du certificat>

For the GPG part we can use this action by crazy-max 😅 : https://github.com/marketplace/actions/import-gpg