[C++] Add ParseJson(), Parser(Parser&&), update fuzzers (#6284)

- add a new method ParseJson to minimize failures during fuzzing
- add default (conditional) move-constructor for Parser
- add a new monster_fuzzer
- switch fuzzers to C++17 and `test/cpp17` generated code

include/flatbuffers/flexbuffers.h

@@ -908,6 +908,11 @@ class Builder FLATBUFFERS_FINAL_CLASS {
     buf_.clear();
   }
 
+#ifdef FLATBUFFERS_DEFAULT_DECLARATION
+  Builder(Builder &&) = default;
+  Builder &operator=(Builder &&) = default;
+#endif
+
   /// @brief Get the serialized buffer (after you call `Finish()`).
   /// @return Returns a vector owned by this class.
   const std::vector<uint8_t> &GetBuffer() const {