New OCI media type

Hi team,

Recently we upgraded our oras tool to v1.1.0 and because of changes in the spec which the tool now supports, our pushes to the registry for OCI format helm artifacts are failing with oras due to the missing support for this media type:

$ oras push -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD --artifact-type application/vnd.cncf.helm.chart.content.v1.tar+gzip registry.gitlab.com/armed-atk/development/helm-charts/microservices/config/common:0.1.0 dummy.json:application/vnd.cncf.helm.config.v1+json global-configs/common.yaml:application/vnd.cncf.helm.chart.content.v1.tar+gzip
WARNING! Using --password via the CLI is insecure. Use --password-stdin.
Exists    ca3d163bab05 dummy.json
Exists    611957de8995 global-configs/common.yaml
Error: PUT "https://registry.gitlab.com/v2/armed-atk/development/helm-charts/microservices/config/common/manifests/0.1.0": response status code 400: manifest invalid: manifest invalid: unknown media type: application/vnd.oci.empty.v1+json

As of OCI spec v1.1.0-rc4, the spec now defines an empty artifact type: application/vnd.oci.empty.v1+json which is "Empty for unused descriptors".

This is a request to add support for this new media type in order to get oras v1.1.0 working. Additionally, it may help if the new spec version can be supported once the stable version is published, however this should not be blocked waiting on the stable version, as we are already experiencing this failure and prefer to not roll back oras to the older release if at all possible.

Possibly related to #1135 (closed) but definitely not caused by it.