code: public <P> RestHandlerBuilder addCustomParamConverter(Class<P> paramClass, ParamConverter<P> converter) {
          return addCustomParamConverterProvider(new ParamConverterProvider() {
              @Override
              public <T> ParamConverter<T> getConverter(Class<T> rawType, Type genericType, Annotation[] annotations) {
                  if (!rawType.equals(paramClass)) {
                      return null;
                  }
                  return (ParamConverter<T>) converter;
              }
          });
      }
docs: <p>Registers a parameter converter class that convert strings to and from a custom class.</p>
  <p>This allows you to specify query string parameters, form values, header params and path params as custom classes.</p>
  <p>For more functionality, {@link #addCustomParamConverterProvider(ParamConverterProvider)} is also available.</p>
  
  @param paramClass The class that this converter is meant for.
  @param converter  The converter
  @param <P>        The type of the parameter
  @return This builder