[json-core] Add JsonExtract helper to ease extracting values and number types from raw Map<String,Object>

[rbygrave]

Helps mostly when extracting values out of nested documents, and dealing with the various number types and type conversion

Example:

  String json = "{\"name\":\"Rob\",\"score\":4.5,\"whenActive\":\"2025-10-20\",\"address\":{\"street\":\"Pall Mall\"}}";
  Map<String, Object> mapFromJson = simpleMapper.fromJsonObject(json);

  JsonExtract extract = simpleMapper.extract(mapFromJson);

  String name = extract.extract("name");
  double score = extract.extract("score", -1D);
  String street = extract.extract("address.street");

  LocalDate activeDate = extract.extractOrEmpty("whenActive")
    .map(LocalDate::parse)
    .orElseThrow();