chore(middleware-flexible-checksums): delay checksum validation until stream read

[trivikr]

Issue

Internal JS-5537

Description

Delays checksum validation until stream read

Testing

import assert from "assert";
import { S3 } from "@aws-sdk/client-s3";
// import { S3 } from "../aws-sdk-js-v3/clients/client-s3/dist-cjs/index.js";

const s3 = new S3({ region: "us-west-2" });
const Bucket = "test-flexible-checksums"; // Change to your bucket name
const Key = "test.txt";
const Body = "helloworld".repeat(10_000_000); // A string of size ~100MB

await s3.putObject({ Bucket, Key, Body, ChecksumAlgorithm: "CRC32" });

const apiCallStart = performance.now();
const response = await s3.getObject({ Bucket, Key, ChecksumMode: "ENABLED" });
console.log("time taken for request:", (performance.now() - apiCallStart).toFixed(2), "ms");

const transformStart = performance.now();
const body = await response.Body.transformToString();
console.log("time taken for transform:", (performance.now() - transformStart).toFixed(2), "ms");

assert(body === Body);

Before

Response checksum is validated when API call is made.

time taken for request: 3141.02 ms
time taken for transform: 221.46 ms

After

Response checksum is validation when stream is consumed (i.e. when transform call is made)

time taken for request: 1217.04 ms
time taken for transform: 2141.58 ms

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[trivikr]

E2E test was added in d9d7534 (#6629)

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