Spring-ai Accept response as a String instead of ChatResponse in EvaluationRequest

This is similar to https://github.com/spring-projects/spring-ai/pull/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);
}

Comment From: markpollack

updated docs merged in b5cc9db88a89356917fc876955ab9a80b5674e5f