add json import path to fix tgc unit test (GoogleCloudPlatform#16178)

Author: zli82016

mmv1/templates/tgc_next/services/resource.go.tmpl

@@ -19,6 +19,7 @@ package {{ lower $.ProductMetadata.Name }}
 import (
   "bytes"
   "context"
+  "encoding/json"
   "fmt"
   "log"
   "reflect"
@@ -46,6 +47,7 @@ var (
     _ = bytes.Clone
     _ = context.WithCancel
     _ = fmt.Sprintf
+    _ = json.Unmarshal
     _ = log.Print
     _ = reflect.ValueOf
     _ = regexp.Match