feat: support --format in manifest fetch command

cmd/oras/internal/display/metadata/json/manifest_fetch.go

@@ -39,8 +39,6 @@ func NewManifestFetchHandler(out io.Writer) metadata.ManifestFetchHandler {
 // OnFetched is called after the manifest fetch is completed.
 func (h *manifestFetchHandler) OnFetched(path string, desc ocispec.Descriptor, content []byte) error {
 	var manifest map[string]any
-	if err := json.Unmarshal(content, &manifest); err != nil {
-		return err
-	}
+	_ = json.Unmarshal(content, &manifest)
 	return printJSON(h.out, model.NewFetched(path, desc, manifest))
 }

cmd/oras/internal/display/metadata/template/manifest_fetch.go

@@ -41,9 +41,6 @@ func NewManifestFetchHandler(out io.Writer, template string) metadata.ManifestFe
 // OnFetched is called after the manifest fetch is completed.
 func (h *manifestFetchHandler) OnFetched(path string, desc ocispec.Descriptor, content []byte) error {
 	var manifest map[string]any
-	if err := json.Unmarshal(content, &manifest); err != nil {
-		return err
-	}
-	// return parseAndWrite(h.out, model.ToMappable(reflect.ValueOf(manifest)), h.template)
+	_ = json.Unmarshal(content, &manifest)
 	return parseAndWrite(h.out, model.NewFetched(path, desc, manifest), h.template)
 }

cmd/oras/root/manifest/fetch.go

@@ -90,9 +90,9 @@ Example - Fetch raw manifest from an OCI layout archive file 'layout.tar':
 
 	cmd.Flags().StringSliceVarP(&opts.mediaTypes, "media-type", "", nil, "accepted media types")
 	cmd.Flags().StringVarP(&opts.outputPath, "output", "o", "", "file `path` to write the fetched manifest to, use - for stdout")
-	cmd.Flags().StringVar(&opts.Template, "format", "", `[Experimental] Format fetched manifest using a custom template:
-'json':       Print manifest in prettified JSON format
-'$TEMPLATE':  Print output using the given Go template.`)
+	cmd.Flags().StringVar(&opts.Template, "format", "", `[Experimental] Format operation metadata using a custom template:
+'json':       Print in prettified JSON format
+'$TEMPLATE':  Print using the given Go template.`)
 	option.ApplyFlags(&opts, cmd.Flags())
 	return oerrors.Command(cmd, &opts.Target)
 }