[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/idl.h

@@ -804,6 +804,11 @@ class Parser : public ParserState {
     }
   }
 
+#ifdef FLATBUFFERS_DEFAULT_DECLARATION
+  Parser(Parser&&) = default;
+  Parser& operator=(Parser&&) = default;
+#endif
+
   // Parse the string containing either schema or JSON data, which will
   // populate the SymbolTable's or the FlatBufferBuilder above.
   // include_paths is used to resolve any include statements, and typically
@@ -818,6 +823,8 @@ class Parser : public ParserState {
   bool Parse(const char *_source, const char **include_paths = nullptr,
              const char *source_filename = nullptr);
 
+  bool ParseJson(const char *json, const char *json_filename = nullptr);
+
   // Set the root type. May override the one set in the schema.
   bool SetRootType(const char *name);
 
@@ -945,6 +952,7 @@ class Parser : public ParserState {
                                     const char **include_paths,
                                     const char *source_filename,
                                     const char *include_filename);
+  FLATBUFFERS_CHECKED_ERROR DoParseJson();
   FLATBUFFERS_CHECKED_ERROR CheckClash(std::vector<FieldDef *> &fields,
                                        StructDef *struct_def,
                                        const char *suffix, BaseType baseType);