[Helm] Add Support for Chart verification using Cosign

[souleb]

Take advantage of PR #876 new oci pkg to provide a chart verification capability to the helmChart reconciler.

[developer-guy]

AFAIK, @stefanprodan is also planning the same thing 🙃

[developer-guy]

would you like to collaborate for this issue? ✌️

/cc @Dentrax

[souleb]

Hello There. We discussed this during the last dev meeting. Let's chat online about pairing.