feat: Support for wrap and unwrap key on web platform

[koji-1009]

close #726

I have added a function to wrap a key stored in LocalStorage using an application specific encryption key. This function uses the WebCrypto API wrapKey and unwrapKey.

The wrapping key can be generated by the following snippet.

async function main() {
  const iv = new Uint8Array(12);
  window.crypto.getRandomValues(iv);

  const key = await window.crypto.subtle.generateKey(
    {
      name: "AES-GCM",
      length: 256,
      iv: iv,
    },
    true,
    ["wrapKey", "unwrapKey"]
  );

  const jsonWebKeyBuffer = await window.crypto.subtle.exportKey("raw", key);
  const jsonWebKey = new Uint8Array(jsonWebKeyBuffer);

  console.log("---iv---");
  const base64Iv = btoa(String.fromCharCode.apply(null, iv));
  console.log(base64Iv);

  console.log("---wrapping key---");
  const wrappingKey = btoa(String.fromCharCode.apply(null, jsonWebKey));
  console.log(wrappingKey);
}

main();

To use the function, set a value to the option as follows.

final _storage = const FlutterSecureStorage(
  // This is example key and iv
  webOptions: WebOptions(
    wrapKey: 'FQQloR/Xwv1BcJOc3VrcxlJatdA/C+n6+v7pFJC0GBs=',
    wrapKeyIv: 'YTVC7K5dOsQTjk+z',
  ),
);

[juliansteenbakker]

Thank you for this function! It would be great if we have some documentation on how to implement this feature in the readme

[koji-1009]

@juliansteenbakker
Thanks for the review! I have reflected the changes in the README.

[juliansteenbakker]

Thank you ! LGTM!