Implement support for the subtle.generateKey operation

[oleiade]

This PR addresses #13 and implements the subtle.generateKey operation.

It implements the function according to the W3C specification, and offers the following API.

The PR contains extensive usage examples offered by the functionalities of this PR, but here's a quick, illustrative example, of an ECDSA key pair generation:

import { crypto } from "k6/x/webcrypto";

export default function () {
  crypto.subtle
    .generateKey({ name: "ECDSA", namedCurve: "P-256" }, true, [
      "sign",
      "verify",
    ])
    .then((key) => {
      console.log(JSON.stringify(key));
    });
}

I'm sure there are many potential improvements possible left for this, and I'm looking forward to hearing from you 🙇🏻

@mostafa, I added you as an optional reviewer; if you have time, could you please take a quick look to ensure I haven't done something wrong on the key generation part? The code for those live in the aes.go, rsa.go, elliptic_curve.go and hmac.go files (GenerateKey functions).

[olegbespalov]

I did an initial round of review, but I believe the current approach should be adjusted by introducing the Algorithm interface, a constructor for specific algorithms, and so on.

[mostafa]

LGTM!