[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

tests/fuzzer/flatbuffers_verifier_fuzzer.cc

@@ -5,7 +5,7 @@
 #include <stdint.h>
 #include <string>
 
-#include "monster_test_generated.h"
+#include "cpp17/generated_cpp17/monster_test_generated.h"
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   flatbuffers::Verifier verifier(data, size);