spec: fetch manifests by tag or digest

Manifests are now fetched by a field called "reference", which may be a tag or
a digest. When using digests to reference a manifest, the data is immutable.
The routes and specification have been updated to allow this.

There are a few caveats to this approach:

1. It may be problematic to rely on data format to differentiate between a tag
   and a digest. Currently, they are disjoint but there may modifications on
   either side that break this guarantee.
2. The caching characteristics of returned content are very different for
   digest versus tag-based references. Digest urls can be cached forever while tag
   urls cannot.

Both of these are minimal caveats that we can live with in the future.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry/api/v2/errors.go

@@ -13,6 +13,9 @@ const (
 	// ErrorCodeUnknown is a catch-all for errors not defined below.
 	ErrorCodeUnknown ErrorCode = iota
 
+	// ErrorCodeUnsupported is returned when an operation is not supported.
+	ErrorCodeUnsupported
+
 	// ErrorCodeUnauthorized is returned if a request is not authorized.
 	ErrorCodeUnauthorized