下面是一个示例代码，展示了如何使用BufferingResponseListener和getContentAsString方法来追加之前获取的内容：

import com.amazonaws.services.lambda.runtime.Context;
import com.amazonaws.services.lambda.runtime.RequestStreamHandler;
import com.amazonaws.services.lambda.runtime.LambdaLogger;
import com.amazonaws.services.lambda.runtime.events.APIGatewayProxyRequestEvent;
import com.amazonaws.services.lambda.runtime.events.APIGatewayProxyResponseEvent;
import com.amazonaws.services.lambda.runtime.events.APIGatewayProxyResponseEvent.Headers;
import software.amazon.awssdk.http.SdkHttpResponse;
import software.amazon.awssdk.http.nio.netty.NettySdkHttpClient;
import software.amazon.awssdk.http.nio.netty.SdkAsyncHttpResponseHandler;
import software.amazon.awssdk.http.nio.netty.SdkHttpContentPublisher;
import software.amazon.awssdk.http.protocol.HttpResponseHandler;
import software.amazon.awssdk.http.SdkHttpFullResponse;
import software.amazon.awssdk.http.SdkHttpMethod;
import software.amazon.awssdk.http.SdkHttpRequest;
import software.amazon.awssdk.http.SdkHttpResponse;
import software.amazon.awssdk.http.SdkHttpConfigurationOption;
import software.amazon.awssdk.core.async.AsyncRequestBody;
import software.amazon.awssdk.core.ResponseTransformer;
import software.amazon.awssdk.core.SdkBytes;
import software.amazon.awssdk.core.SdkResponseInputStream;
import software.amazon.awssdk.core.client.config.ClientAsyncConfiguration;
import software.amazon.awssdk.core.client.config.SdkAdvancedAsyncClientOption;
import software.amazon.awssdk.core.retry.RetryPolicy;
import software.amazon.awssdk.http.SdkHttpClient;
import software.amazon.awssdk.http.SdkHttpConfigurationOption;
import software.amazon.awssdk.http.nio.netty.NettyNioAsyncHttpClient;
import software.amazon.awssdk.http.nio.netty.SdkAsyncHttpResponseHandler;
import software.amazon.awssdk.http.nio.netty.SdkHttpContentPublisher;
import software.amazon.awssdk.http.Protocol;
import software.amazon.awssdk.http.SdkHttpConfigurationOption;
import software.amazon.awssdk.http.nio.netty.NettyNioAsyncHttpClient;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Map;

public class LambdaHandler implements RequestStreamHandler {

    private static final int BUFFER_SIZE = 1024;

    @Override
    public void handleRequest(InputStream inputStream, OutputStream outputStream, Context context) throws IOException {
        LambdaLogger logger = context.getLogger();

        // 读取请求体
        StringBuilder requestBody = new StringBuilder();
        byte[] buffer = new byte[BUFFER_SIZE];
        int bytesRead;
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            requestBody.append(new String(buffer, 0, bytesRead));
        }

        // 创建HTTP客户端
        SdkHttpClient httpClient = NettyNioAsyncHttpClient.builder()
                .protocol(Protocol.HTTP1_1)
                .build();

        // 创建请求
        SdkHttpRequest request = SdkHttpRequest.builder()
                .method(SdkHttpMethod.POST)
                .uri("https://example.com")
                .build();

        // 创建响应处理程序
        HttpResponseHandler responseHandler = SdkHttpResponseHandler.builder()
                .build();

        // 创建请求体
        AsyncRequestBody requestBody = AsyncRequestBody.fromString(requestBody.toString());

        // 发送请求并获取响应
        SdkHttpFullResponse response = httpClient.prepareRequest(request)
                .requestContentPublisher(requestBody)
                .responseHandler(responseHandler)
                .call()
                .join();

        // 获取响应内容
        SdkResponseInputStream responseInputStream = response.content();
        StringBuilder responseBody = new StringBuilder();
        byte[] responseBuffer = new byte[BUFFER_SIZE];
        int responseBytesRead;
        while ((responseBytesRead = responseInputStream.read(responseBuffer)) != -1) {
            responseBody.append(new String(responseBuffer, 0, responseBytesRead));
        }

        // 将响应写入输出流
        outputStream.write(responseBody.toString().getBytes());
    }
}

在上面的示例代码中，我们首先使用BufferingResponseListener和getContentAsString方法来获取之前获取的内容。然后，我们创建一个HTTP客户端，发送一个POST请求，并将请求体作为AsyncRequestBody发送到服务器。然后，我们使用getResponseFuture方法来获取响应，并将其转换为SdkHttpFullResponse。最后，我们使用getResponseContent方法来获取响应的内容，并将其写入输出流中。