Arbitrary binary HTTP request bodies

[na--]

Currently users can read a binary file or receive a binary HTTP response and send that as an HTTP request body:

import { sleep } from "k6";
import http from "k6/http";
import crypto from "k6/crypto";

// Manually downloaded from https://httpbin.org/image/png
var binaryFile = open("./image.png", "b");

export default function (data) {
    let binaryResp = http.get("https://httpbin.org/image/png", { responseType: "binary" });

    console.log(crypto.sha256(binaryFile, "hex"));
    console.log(crypto.sha256(binaryResp.body, "hex"));
    console.log("Lengths: ", binaryFile.length, binaryResp.body.length);

    let resp = http.post("https://httpbin.org/anything", binaryResp.body);
    console.log(resp.body);
}

They can even modify those binary arrays without extending them (so binaryFile[100] = 123; will work, but binaryFile[10000] = 123; won't). But they cannot construct a new binary buffer from scratch. Normal JS arrays won't (and probably shouldn't) work, and JS strings are unsuitable for a lot of tasks, since they're UTF-16 encoded. Ideally, once we fix this issue, we can use typed arrays for sending arbitrary binary payloads as HTTP request bodies.

[na--]

This was fixed by #1776, which allowed users to pass ArrayBuffer values as the request body. Everything else that's binary-related is tracked by #1020 and has a WIP PR already (#1800).