MoneroWalletKeys.createSubaddress() not supported
Heyo there! 
So we have a `MoneroWalletKeys` (view only wallet) on the client side and want to continously generate subaddresses with it. Intuitively we wanted to use `.createSubaddress()` but then got the error that it is not supported. 

Looking at the `src/main/ts/wallet/MoneroWallet.ts` it looks like this is not implemented, just for the `MoneroWalletFull`. We are wondering if the function of the `MoneroWalletFull` could not also be adopted for the `MoneroWalletKeys`. 

I guess a workable workaround is to use `MoneroWalletKeys.getAddress()` and then keep track of the subaddress index elsewhere.

Cheers!