Go で S3 オブジェクトを効率的にダウンロードするために io.WriterAt を実装するにはどうすればよいですか?

import (
    "context"
    "fmt"

    "github.com/aws/aws-sdk-go/aws"
    "github.com/aws/aws-sdk-go/aws/session"
    "github.com/aws/aws-sdk-go/service/s3"
)

func main() {
    // Create a new session object with the default options.
    sess := session.Must(session.NewSession())

    // Create a new S3 client.
    client := s3.New(sess)

    // Prepare the request input with the bucket and key of the object to download.
    requestInput := &s3.GetObjectInput{
        Bucket: aws.String("my-bucket"),
        Key:    aws.String("my-file"),
    }

    // Create a new aws.WriteAtBuffer to receive the downloaded object data.
    buf := aws.NewWriteAtBuffer([]byte{})

    // Download the object into the buffer.
    if _, err := client.GetObjectWithContext(context.Background(), requestInput, buf); err != nil {
        fmt.Printf("Error downloading file: %v", err)
        return
    }

    // Print the number of bytes downloaded.
    fmt.Printf("Downloaded %v bytes", len(buf.Bytes()))
}