Introduce hpb::Arena stub

PiperOrigin-RevId: 671509864

hpb/BUILD

@@ -170,3 +170,12 @@ cc_library(
         "//upb:mini_table",
     ],
 )
+
+cc_library(
+    name = "arena",
+    hdrs = ["arena.h"],
+    visibility = ["//visibility:public"],
+    deps = [
+        "//upb:mem",
+    ],
+)

hpb/arena.h

@@ -0,0 +1,10 @@
+#ifndef GOOGLE_PROTOBUF_HPB_ARENA_H__
+#define GOOGLE_PROTOBUF_HPB_ARENA_H__
+
+#include "upb/mem/arena.hpp"
+
+namespace hpb {
+using Arena = upb::Arena;
+}
+
+#endif  // GOOGLE_PROTOBUF_HPB_ARENA_H__