Accept response as a String instead of ChatResponse in EvaluationRequest

[habuma]

This is similar to #948, but applied to EvaluatorRequest. Aside from being about to use the evaluator at runtime, this makes it possible to write a test like this:

@Test
public void testAsk() {
  String question = "Why is the sky blue?";
  Answer answer = askService.ask(new Question(question));

  RelevancyEvaluator evaluator = new RelevancyEvaluator(chatClientBuilder);
  EvaluationResponse response = evaluator.evaluate(
      new EvaluationRequest(
          question,
          List.of(),
          "Scattering of light causes the sky to be blue."));

  Assertions.assertThat(response.getScore()).isEqualTo(1.0f);
}

[markpollack]

updated docs
merged in b5cc9db