aws/retry: Update Retry middleware to save last attempt ResultMetadata

[jasdel]

Previously all attempt metadata was being nested into the AttemptResults key and a new metadata was being returned with only that item. This change ensures that the returned metadata includes all keys from the last attempt's metadata, along with the AttemptResults.

Depends on aws/smithy-go#318

Fixes #1154

Example usage of GetRawResponse to get the smithyhttp.Response value from the metadata.

package main

import (
	"context"
	"flag"
	"log"

	"github.com/aws/aws-sdk-go-v2/aws"
	"github.com/aws/aws-sdk-go-v2/aws/middleware"
	"github.com/aws/aws-sdk-go-v2/config"
	"github.com/aws/aws-sdk-go-v2/service/s3"
	smithyhttp "github.com/aws/smithy-go/transport/http"
)

func main() {
	var bucket, key string
	flag.StringVar(&bucket, "b", "", "The bucket name")
	flag.StringVar(&key, "k", "", "The key name")
	flag.Parse()

	cfg, err := config.LoadDefaultConfig(context.Background())
	if err != nil {
		log.Fatalf("expect no error, %v", err)
	}

	client := s3.NewFromConfig(cfg)

	headObjResp, err := client.HeadObject(context.Background(), &s3.HeadObjectInput{
		Bucket: &bucket,
		Key:    &key,
	})
	if err != nil {
		log.Fatalf("expect no error, %v", err)
	}

	rawResp := middleware.GetRawResponse(headObjResp.ResultMetadata)
	httpResp, ok := rawResp.(*smithyhttp.Response)
	if !ok {
		log.Fatalf("expect http response, got %T, %#v", rawResp, rawResp)
	}

	log.Printf("tagging count: %v", httpResp.Header.Get("X-Amz-Tagging-Count"))
}