Add --unreferenced-only flag to acr purge

I can use `acr purge` to delete some manifests.
I can add either `--untagged` or `--untagged-only` flag to limit the delete to manifests which don't have any tag.
Thsi si ok.

But sometimes I have manifests that have no tags, but they have a referrer artifact (in a OCI artifact graph).
And I would like avoiding to delete this manifests.

This is the reason why I ask for a `--unreferenced-only` flag (to be used alone or together with `--untagged` or `--untagged-only`) to delete only manifests which don't have any referrer artifacts.