RPC commands require token to be set in client options

For RPC calls when `wallet-auth: true`, the admin token needs to be set as an option when instantiating the client:

```
const {WalletClient, Network} = require('bcoin');
const network = Network.get('regtest');

const walletOptions = {
network: network.type,
port: network.walletPort,
apiKey: 'api-key'
token:
}

const walletClient = new WalletClient(walletOptions);

(async () => {
const result = await walletClient.execute('getwalletinfo');
console.log(result);
})();
```

The `token` property is stored in the `bcurl/client`, without setting this property there is no other way to add a token to an RPC call -- note that with the REST-API HTTP calls the docs already specify to add `?token=...` or `--token=...`

https://bcoin.io/api-docs/?shell--curl#wallet-authentication

Start with the linked API documentation's wallet-authentication section and inspect how the existing REST API token guidance is presented. Document that RPC WalletClient calls require the token option, using the reported example as a guide; done when the RPC authentication instructions clearly show how to supply the token.

Written by the indexing model from the issue text.