[json-core] Add SimpleMapper API to core to make it easier for reading/writing simple Maps, Lists etc

[rbygrave]

 static final SimpleMapper simpleMapper = SimpleMapper.builder().build();

  void test() {

    Map<String, Long> map = new LinkedHashMap<>();
    map.put("one", 45L);
    map.put("two", 93L);

    String asJson = simpleMapper.toJson(map);
    assertThat(asJson).isEqualTo("{\"one\":45,\"two\":93}");

    Map<String, Object> mapFromJson = simpleMapper.fromJsonObject(asJson);

    assertThat(mapFromJson).containsKeys("one", "two");
    assertThat(mapFromJson.toString()).isEqualTo("{one=45, two=93}");
  }