Add ImageLayoutVersion const

Add a ImageLayoutVersion = 1.0.0 const so implementation can
refer to rather than hard code in implementation just like
https://github.com/containers/image/blob/master/oci/layout/oci_dest.go#L173
```
if err := ioutil.WriteFile(d.ref.ociLayoutPath(), []byte(`{"imageLayoutVersion": "1.0.0"}`), 0644); err != nil {
		return err
}
```

Signed-off-by: Lei Jitang <[email protected]>

Author: coolljt0725

specs-go/v1/layout.go

@@ -14,6 +14,8 @@
 
 package v1
 
+const ImageLayoutVersion = "1.0.0"
+
 // ImageLayout is the structure in the "oci-layout" file, found in the root
 // of an OCI Image-layout directory.
 type ImageLayout struct {