Introduce CreateMessage w/ default-init msg inside hpb::interop::upb

PiperOrigin-RevId: 676071677

hpb/backend/upb/interop.h

@@ -95,6 +95,18 @@ typename T::Proxy MakeHandle(upb_Message* msg, upb_Arena* arena) {
   return typename T::Proxy(msg, arena);
 }
 
+/**
+ * Creates a message in the given arena and returns a handle to it.
+ *
+ * The supplied arena must outlive the hpb handle.
+ * All messages reachable from from the upb message must
+ * outlive the hpb handle.
+ */
+template <typename T>
+typename T::Proxy CreateMessage(upb_Arena* arena) {
+  return hpb::internal::PrivateAccess::CreateMessage<T>(arena);
+}
+
 inline absl::string_view FromUpbStringView(upb_StringView str) {
   return absl::string_view(str.data, str.size);
 }