Initial PR to propose updates to tag listing

Signed-off-by: Jeff Carter <[email protected]>

Author: jcarter3

spec.md

@@ -559,10 +559,52 @@ 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`
+
+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...",
+      "annotations": {
+        "org.opencontainers.image.ref.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...",
+      "annotations": {
+        "org.opencontainers.image.ref.created": "2022-01-01T07:21:33Z",
+        "org.opencontainers.image.ref.name": "<tag2>"
+      }
+    }
+  ]
+}
+```
+The following annotations MUST be included:
+* `org.opencontainers.image.ref.created` - Date and time the reference was created (date-time string as defined by RFC 3339). If unknown, default to epoch.
+* `org.opencontainers.image.ref.name` - Reference name of the manifest.
+
+The following annotations MAY be included:
+* `org.opencontainers.image.created` - Date and time on which the image was built (date-time string as defined by RFC 3339).
+* `org.opencontainers.image.pull.count` - Number of times the image has been pulled from the registry.
+* `org.opencontainers.image.pull.timestamp` - Date and time on which the image was last pulled from the registry.
+
 _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.
-When available, clients should prefer the `Link` header over using the `last` parameter for pagination.
+When available, clients should prefer the `Link` header over using the `last` parameter for pagination. If using enhanced
+tag listing, the `Link` header MUST be provided.
 
 ##### Listing Referrers