Add reference types support to distribution.

[sajayantony]

• Make and run the registry with the sample config in the repo

./bin/registry serve cmd/registry/config-example-with-extensions.yml

• You can view the manifests using a test extension

➜ http -b :5000/v2/reftype-test/_distribution/registry/manifests
{
    "digests": [
        "sha256:28ffe613ed468b4aa3b60061f529ef676144d47bfc7067bd465d58e370f07718",
        "sha256:65b3a80ebe7471beecbc090c5b2cdd0aafeaefa0715f8f12e40dc918a3a70e32"
    ],
    "name": "reftype-test"
}

In the above, digest 65b3a80 is an image and 28ffe6 refers to that image. You can query the first digest for things that refer to it.

➜ http -b 'localhost:5000/v2/reftype-test/_oci/artifacts/referrers?digest=sha256:65b3a80ebe7471beecbc090c5b2cdd0aafeaefa0715f8f12e40dc918a3a70e32'
{
    "manifests": [
        {
            "digest": "sha256:28ffe613ed468b4aa3b60061f529ef676144d47bfc7067bd465d58e370f07718",
            "mediaType": "application/vnd.oci.artifact.manifest.v1+json",
            "size": 491
        }
    ],
    "schemaVersion": 2
}

• Here is the manifest for the artifact using the artifact manifest defined -

https://github.com/oci-playground/artifact-spec/blob/8fed0a29d06fc27c88c787722707af59499c30b5/specs-go/v1/artifact.go#L21-L33

[jdolitsky]

I'm curious about how this compares to Proposal E as it stands, for example /referrers vs /references, but I'm in favor of getting things merged quickly into this fork

[sajayantony]

Good catch @jdolitsky - I'm ok merging and updating as well since the names will change here -
opencontainers/wg-reference-types#44

[jdolitsky]

See also #2
Would be great to get the demo steps from the PR description directly in the README here