Allow clients to specify the request date on signing requests

[xeviknal-cf]

The main intent of this PR is to make the signed urls api flexible with regards to the time when it's created.

GOAL: implement what it's called Cacheable signed URLs. Using v4 presign method, the current Date is necessary. Meaning that we can't generate constant signed urls according to our needs.

example: we want to avoid certain customers to download again and again the same set of images. And we don't want to make those images publicly available. If those images have a Control-Cache: private, max-age=3600 policy, it won't be used since each time we sign URLs those will be completely different. Therefore the browser won't be able to re-use them.

original idea: https://advancedweb.hu/cacheable-s3-signed-urls/

[lucix-aws]

We recently announced the upcoming end-of-life for SDK v1 and are no longer accepting PRs for additive features. I had meant to update the PR template stating as such, I'll do that now.

What you're trying to do here is innately possible in SDK v2 through wrapping the HTTPPresignerV4 interface:

package main

import (
	"context"
	"fmt"
	"net/http"
	"time"

	"github.com/aws/aws-sdk-go-v2/aws"
	v4 "github.com/aws/aws-sdk-go-v2/aws/signer/v4"
	"github.com/aws/aws-sdk-go-v2/config"
	"github.com/aws/aws-sdk-go-v2/service/s3"
)

type timedPresigner struct {
	signingTime time.Time
	signer      s3.HTTPPresignerV4
}

func (t *timedPresigner) PresignHTTP(
	ctx context.Context, creds aws.Credentials, r *http.Request, payloadHash, service, region string, _ time.Time, opts ...func(*v4.SignerOptions),
) (string, http.Header, error) {
	return t.signer.PresignHTTP(ctx, creds, r, payloadHash, service, region, t.signingTime, opts...)
}

func main() {
	cfg, err := config.LoadDefaultConfig(context.Background())
	if err != nil {
		panic(err)
	}

	svc := s3.NewFromConfig(cfg)
	psvc := s3.NewPresignClient(svc, func(o *s3.PresignOptions) {
		o.Presigner = &timedPresigner{
			signingTime: time.UnixMilli(1000),
			signer:      v4.NewSigner(),
		}
	})

	req, err := psvc.PresignGetObject(context.Background(), &s3.GetObjectInput{
		Bucket: aws.String("bucket"),
		Key:    aws.String("key"),
	})
	if err != nil {
		panic(err)
	}

	fmt.Println(req.URL)
}