How to append arbitrary data to tx-extra?
Hi, I have been trying to create a tx with arbitrary data in the `NONCE` field of tx-extra.

So far I tried `getEtxra` and  `setExtra` functions(to get the already populated extra and append what I need and set it back) on the `MoneroTxWallet` type. But `getEtxra` returns `undefined` on a `createTx` result. And when just call `setExtra` then extra only becomes the extra I set, so tx becomes invalid.

 Is there a way to do that using this lib? Here is little snippet of what I tried:

```ts
    // create the tx
    const tx = await this.wallet.createTx({
      address: to,
      amount: BigInt(amount_to_send),
      accountIndex: 0,
      relay: false,
    });

    // update tx-extra
    const NONCE_FIELD = Buffer.from([2]);
    const new_extra = Buffer.concat([tx.getExtra(), NONCE_FIELD, data]);
    tx.setExtra(new_extra);

    return this.wallet.relayTx(tx);
```

this returns `Failed to commit tx` error. Because when I print the tx I see there is an "extra" field now(that didn't exist before hence I was getting `undefined`) that is equal to `data` I set.

Thanks in advance.