Initial PR to propose updates to tag listing

Signed-off-by: Jeff Carter <jeff.carter@docker.com>

Author: jcarter3

spec.md

@@ -559,6 +559,44 @@ The tags MUST be in lexical order.
 
 When using the `last` query parameter, the `n` parameter is OPTIONAL.
 
+Registries MAY support an enhanced tag listing experience when clients pass an `Accept` header value of 
+`application/vnd.oci.image.index.v1+json`. If a registry supports this mode, they MUST support these additional query parameters:
+* `sort_by` - allowed values are `created_at`, `name`
+* `sort_order` - allowed values are `asc` and `desc`
+* `artifact_type` - Artifact types to include in response, can be specified multiple times. If not set, do not return artifacts.
+
+The `last` query parameter MUST contain a value consistent with the specified `sort_by`.
+
+The response MUST be a json body in the following format:
+```json
+{
+  "schemaVersion": 2,
+  "mediaType": "application/vnd.oci.image.index.v1+json",
+  "manifests": [
+    {
+      "mediaType": "application/vnd.oci.image.manifest.v1+json",
+      "size": 1234,
+      "digest": "sha256:a1a1a1...",
+      "artifactType": "application/vnd.example.sbom.v1",
+      "annotations": {
+        "org.opencontainers.image.created": "2022-01-01T14:42:55Z",
+        "org.opencontainers.image.ref.name": "<tag1>"
+      }
+    },
+    {
+      "mediaType": "application/vnd.oci.image.manifest.v1+json",
+      "size": 1234,
+      "digest": "sha256:a2a2a2...",
+      "artifactType": "application/vnd.example.signature.v1",
+      "annotations": {
+        "org.opencontainers.image.created": "2022-01-01T07:21:33Z",
+        "org.opencontainers.image.ref.name": "<tag2>"
+      }
+    }
+  ]
+}
+```
+
 _Implementers note:_
 Previous versions of this specification did not include the `Link` header.
 Clients depending on the number of tags returned matching `n` may prematurely stop pagination on registries using the `Link` header.