Expose logic to serialize a DeltaArchiveManifest to proper payload file

So that unittests can construct a cumstom manifest and use that logic to
create a valid payload file

Test: th
Change-Id: Ie6ba00b7fafc12eb250787295c93f966af1f376f
diff --git a/payload_generator/payload_file.h b/payload_generator/payload_file.h
index 3a45793..86bf243 100644
--- a/payload_generator/payload_file.h
+++ b/payload_generator/payload_file.h
@@ -56,6 +56,13 @@
                     const std::string& private_key_path,
                     uint64_t* metadata_size_out);
 
+  static bool WritePayload(const std::string& payload_file,
+                           const std::string& ordered_blobs_file,
+                           const std::string& private_key_path,
+                           uint64_t major_version_,
+                           const DeltaArchiveManifest& manifest,
+                           uint64_t* out_metadata_size);
+
  private:
   FRIEND_TEST(PayloadFileTest, ReorderBlobsTest);