Support for OCI artifacts

`rules_oci` provides good support for OCI images already. Other OCI artifacts like helm charts and [ORAS](https://oras.land/) are becoming more and more popular. A natural extension for `rules_oci` is adding support for building and pushing generic OCI artifacts. To keep the scope limited it's sufficient to support the [OCI Distribution Spec](https://github.com/opencontainers/distribution-spec/blob/main/spec.md)/[OCI Artifacts Spec](https://github.com/opencontainers/artifacts) and leave application specifics up to 3rd party rules.

If support for OCI artifacts is in scope for `rules_oci` then I'd be happy for some pointers so I can start with a POC. It certainly looks like zot [supports](https://zotregistry.io/v1.4.3/user-guides/user-guide-datapath/) generic OCI artifacts already.

Start by reading the existing rules_oci image support, then review the linked OCI Distribution and OCI Artifacts specifications and the ORAS and zot references. Done means rules_oci can build and push generic OCI artifacts within those specifications, while application-specific behavior remains available to third-party rules.

Written by the indexing model from the issue text.