Implement "idn-hostname" format validator

[GuillaumeBlanchet]

idn-hostname is part of the json-schema specifications: https://json-schema.org/understanding-json-schema/reference/string.html#format

One could implement the feature like that:

public class IdnHostNameValidator implements FormatValidator {

  private static final IDNA idna = IDNA.getUTS46Instance(IDNA.NONTRANSITIONAL_TO_ASCII
      | IDNA.NONTRANSITIONAL_TO_UNICODE
      | IDNA.CHECK_BIDI
      | IDNA.CHECK_CONTEXTJ
      | IDNA.CHECK_CONTEXTO
      | IDNA.USE_STD3_RULES);

  @Override
  public Optional<String> validate(final String domain) {
    StringBuilder asciiDomain = new StringBuilder();
    IDNA.Info info = new IDNA.Info();
    idna.nameToASCII(domain, asciiDomain, info);
    if (info.hasErrors()) {
      return Optional.of(info.getErrors().toString());
    }
    return Optional.empty();
  }
}

parametrizing the icu library from unicode (https://mvnrepository.com/artifact/com.ibm.icu/icu4j) with domain constraints