feat: Add metadata support to PromptUserSpec and PromptSystemSpec in ChatClient

[YunKuiLu]

• Add metadata methods to PromptUserSpec and PromptSystemSpec interfaces
• Update DefaultChatClientRequestSpec to handle user and system metadata
• Modify DefaultChatClientUtils to include metadata in SystemMessage and UserMessage builders
• Add comprehensive test coverage for metadata functionality

---

This PR allows users to set the metadata field for UserMessage and SystemMessage through Consumer<PromptUserSpec> and Consumer<PromptSystemSpec>.

Users can set information like message ID through metadata, and then get them in ChatMemoryRepository.
Example as follows:

  private static final ChatMemoryRepository myChatMemoryRepository = new ChatMemoryRepository() {
      @Override
      public void saveAll(String conversationId, List<Message> messages) {
          log.info("messages={}", messages);
          log.info("id={}", messages.getLast().getMetadata().get("id"));
      }
  };
  
  @GetMapping("/ai/tool")
  public String tool() {
      String userMessageId = UUID.randomUUID().toString();
  
      ChatResponse result = chatClient.prompt()
              .system(s -> s.text("You are a helpful assistant.")
                           // Allow setting metadata for SystemMessage.
                           .metadata("version", "v1.0"))
              .tools(this)
              .user(u -> u.text("What is the weather in New York?")
                          // Allow setting metadata for UserMessage.
                          .metadata("id", userMessageId))
              .advisors(MessageChatMemoryAdvisor.builder(
                      MessageWindowChatMemory.builder()
                              .chatMemoryRepository(myChatMemoryRepository)
                              .maxMessages(10).build()
              ).build())
              .call()
              .chatResponse();
  
      return result.getResult().getOutput().getText();
  }

[sobychacko]

@YunKuiLu Merged upstream. Also, I added a section in the reference documentation for this feature. See this commit: c328ef6