如何更改ResponseEntity的消息字段格式？

import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpStatus;
import org.springframework.http.MediaType;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.http.server.ServerHttpResponse;
import org.springframework.http.server.ServletServerHttpResponse;
import org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice;

import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.List;

public class CustomResponseBodyAdvice implements ResponseBodyAdvice<Object> {
    
    private final MappingJackson2HttpMessageConverter converter;
    
    public CustomResponseBodyAdvice(MappingJackson2HttpMessageConverter converter) {
        this.converter = converter;
    }

    @Override
    public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
        return true;
    }

    @Override
    public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType,
            Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request,
            ServerHttpResponse response) {
        HttpHeaders headers = response.getHeaders();
        headers.setContentType(MediaType.APPLICATION_JSON);
        return body;
    }
}

import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

import java.util.List;

@Configuration
public class CustomWebMvcConfig implements WebMvcConfigurer {

    private final MappingJackson2HttpMessageConverter converter;

    public CustomWebMvcConfig(MappingJackson2HttpMessageConverter converter) {
        this.converter = converter;
    }

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        converters.add(converter);
    }
    
    @Override
    public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
        converters.add(converter);
    }
}

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class ExampleController {

    @Autowired
    private MappingJackson2HttpMessageConverter converter;

    @GetMapping("/example")
    public ResponseEntity<String> example() {
        // 构造响应数据
        String response = "{\"key\":\"value\"}";

        // 创建自定义消息转换器
        CustomResponseBodyAdvice advice = new CustomResponseBodyAdvice(converter);

        // 构造ResponseEntity并返回
        return advice.beforeBodyWrite(response, null, null, null, null, null);
    }
}