feat: PGP Signed Helm Chart

[mattfarina]

Helm has a feature to sign and verify charts so that the provenance and integrity can be checked. Given that kubewarden is around security, this could be useful to use.

There is a plugin to make this easier with GPG.

On Artifact Hub there is a badge to show the provenance file. Here's an example. The badge shows up in search, too.

[viccuad]

Just in case, we have been signing the Helm charts with Sigstore, and are published signed in ghcr.io.

See https://docs.kubewarden.io/tutorials/verifying-kubewarden.