remotes: add FetcherByDigest for fetching blobs without foreknown descriptors

Fetching blobs without foreknown descriptors is useful for using a registry as a general-purpose CAS.

Related: `oras blob fetch` (ORAS v0.15.0)

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

remotes/resolver.go

@@ -21,6 +21,7 @@ import (
 	"io"
 
 	"github.com/containerd/containerd/content"
+	"github.com/opencontainers/go-digest"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
@@ -50,12 +51,23 @@ type Resolver interface {
 	Pusher(ctx context.Context, ref string) (Pusher, error)
 }
 
-// Fetcher fetches content
+// Fetcher fetches content.
+// A fetcher implementation may implement the FetcherByDigest interface too.
 type Fetcher interface {
 	// Fetch the resource identified by the descriptor.
 	Fetch(ctx context.Context, desc ocispec.Descriptor) (io.ReadCloser, error)
 }
 
+// FetcherByDigest fetches content by the digest.
+type FetcherByDigest interface {
+	// FetchByDigest fetches the resource identified by the digest.
+	//
+	// FetcherByDigest usually returns an incomplete descriptor.
+	// Typically, the media type is always set to "application/octet-stream",
+	// and the annotations are unset.
+	FetchByDigest(ctx context.Context, dgst digest.Digest) (io.ReadCloser, ocispec.Descriptor, error)
+}
+
 // Pusher pushes content
 type Pusher interface {
 	// Push returns a content writer for the given resource identified