code gen flexbuffer verifier (#7207)

* code gen flexbuffer verifier

* remove verify nested flexbuffers from flexbuffers

* made function static, and placed higher in file

* moved function to own header

include/flatbuffers/flex_flat_util.h

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2022 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef FLATBUFFERS_FLEX_FLAT_UTIL_H_
+#define FLATBUFFERS_FLEX_FLAT_UTIL_H_
+
+#include "flatbuffers/flatbuffers.h"
+#include "flatbuffers/flexbuffers.h"
+
+namespace flexbuffers {
+
+// Verifies the `nested` flexbuffer within a flatbuffer vector is valid.
+inline bool VerifyNestedFlexBuffer(
+    const flatbuffers::Vector<uint8_t> *const nested,
+    flatbuffers::Verifier &verifier) {
+  if (!nested) return true;
+  return verifier.Check(flexbuffers::VerifyBuffer(
+      nested->data(), nested->size(), verifier.GetFlexReuseTracker()));
+}
+
+}  // namespace flexbuffers
+
+#endif  // FLATBUFFERS_FLEX_FLAT_UTIL_H_