Basic support for structured outputs

[bertilmuth]

This is a resolution to #54.

I tried to stick as close to the OpenAI API as possible.
Note that for the JSON schema generation to work, all properties of the serialized classes must be marked with @NotNull or @JsonProperty(required = true)

Here's a test case that replicates the math tutor example from the OpenAI website:

@Test
  void createChatCompletionWithJsonSchema() throws JsonProcessingException {
      final List<ChatMessage> messages = new ArrayList<>();
      final ChatMessage systemMessage = new SystemMessage("You are a helpful math tutor. Guide the user through the solution step by step.");
      final ChatMessage userMessage = new UserMessage("how can I solve 8x + 7 = -23");
      messages.add(systemMessage);
      messages.add(userMessage);

      Class<MathReasoning> rootClass = MathReasoning.class;
      ChatResponseFormat responseFormat = ChatResponseFormat.jsonSchema(rootClass);
      
      ChatCompletionRequest chatCompletionRequest = ChatCompletionRequest
              .builder()
              .model("gpt-4o-2024-08-06")
              .messages(messages)
              .responseFormat(responseFormat)
              .maxTokens(1000)
              .build();

      ChatCompletionChoice choice = service.createChatCompletion(chatCompletionRequest).getChoices().get(0);
      MathReasoning mathReasoning = choice.getMessage().parsed(rootClass);
      
      String finalAnswer = mathReasoning.getFinal_answer();
      assertTrue(finalAnswer.contains("x"));
      assertTrue(finalAnswer.contains("="));
  }
  
  @Data
  @NoArgsConstructor
  private static class MathReasoning {
      @NotNull private List<Step> steps;
      @NotNull private String final_answer;
  }
  
  @Data
  @NoArgsConstructor
  private static class Step {
      @NotNull private String explanation;
      @NotNull private String output;
  }