[native][AES] Implement generateKey function

Summary:
This diff implements a function, that generates AES-256 key using platforms' native APIs.

iOS docs: https://developer.apple.com/documentation/cryptokit/aes/gcm/nonce
Android docs: https://developer.android.com/reference/kotlin/javax/crypto/KeyGenerator

Depends on D7002

Test Plan:
Ensured the function returns random numbers on both platforms:

import { generateKey } from './utils/aes-crypto-module.js';

console.log(generateKey());
// should display sth like: [82, 226, ..., 215, 19, 13] // 32 values

Reviewers: marcin, atul, varun

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7004