Support additional signing mechanisms than PGP

[scottrigby]

Helm currently supports PGP. There has been discussion about supporting additional signing mechanisms.

Andy volunteered to contribute to this HIP.

May relate to https://github.com/helm/community/blob/main/hips/hip-0015.md

[FrenchBen]

related work (signing without bundling)

helm/helm#10184

[kfox1111]

Anything related to cosign planned? might compliment the oci support nicely.

[sabre1041]

@kfox1111 yes, integrating the sigstore project would be one of the signing options