docs: public <T> JaversBuilder registerValueWithCustomToString(Class<T> valueClass, Function<T, String> toString) {
          argumentsAreNotNull(valueClass, toString);
  
          if (!clientsClassDefinitions.containsKey(valueClass)){
              registerType(new ValueDefinition(valueClass));
          }
          ValueDefinition def = getClassDefinition(valueClass);
          def.setToStringFunction((Function)toString);
  
          return this;
      }