[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

.gitignore

@@ -123,6 +123,7 @@ dart/doc/api/
 Cargo.lock
 .corpus**
 .seed**
+.crash**
 grpc/google/
 **/Package.resolved
 .clangd/**