spec-go/: provide variable of the scratch config digest

to facilitate use by tools that import this reference code to identify
this specific blob when working with an empty/scratch config blob

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

specs-go/v1/manifest.go

@@ -36,3 +36,17 @@ type Manifest struct {
 	// Annotations contains arbitrary metadata for the image manifest.
 	Annotations map[string]string `json:"annotations,omitempty"`
 }
+
+// ScratchConfigDigestSHA256 is the digest of a blob with content of `{}` (size of 2)
+//
+// Example: here is a SCRATCH descriptor JSON document that can be used as the value of a `config` in a manifest:
+// ```json
+//
+//	{
+//	 "mediaType": "application/vnd.oci.image.config.v1+json",
+//	 "size": 2,
+//	 "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a"
+//	}
+//
+// ```
+const ScratchConfigDigestSHA256 = "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a"