make the Wallet more extensible.

[drunkirishcoder]

I would like to implement a wallet that signs remotely and be able to integrate with xrpl. something like

const wallet = new MyRemoteWallet(...)

const res = await client.submitAndWait({
    TransactionType: 'Payment',
    Account: wallet.address,
    Destination: "r...",
    Amount: '1',
  },
  {
    autofill: true,
    wallet,
  })

I can extend my remote wallet from the xrpl wallet, export class MyRemoteWallet extends xrpl.Wallet, but the problem is the sign function is not async, therefore I can't make a remote call from it. though because submitAndWait is async, it should be able to support an async sign function.

essentially if this block of code can be changed to

  let tx =
    typeof transaction === 'string'
      ? // eslint-disable-next-line @typescript-eslint/consistent-type-assertions -- converts JsonObject to correct Transaction type
        (decode(transaction) as unknown as Transaction)
      : transaction

  if (autofill) {
    tx = await client.autofill(tx)
  }

  return (await wallet.sign(tx)).tx_blob

everything would work. this change would make extending xrpl with remote wallets much easier.