Support for legacy referrer API

[jonesbusy]

What would you like to be added?

Some registries like ghcr continue to use https://specs.opencontainers.org/distribution-spec/#unavailable-referrers-api

Add fallback support for <alg>-<ref> format

Add an integration test with https://github.com/jonesbusy/alpine-signed/pkgs/container/alpine-signed

Why is this needed for ORAS?

Support legacy registries

Are you willing to submit PRs to contribute to this feature?

• Yes, I am willing to implement it.