[Feature] - Add support for Discount configuration

## Motivation 

- Enable developers to manage discounts via Javascript API

### Draft API

We've in mind the following API:

```javascript
const payment = await MercadoPagoCheckout.createPayment({
    publicKey: Env.MP_PUBLIC_KEY,
    preferenceId: `your_preference_id`,
    discountOptions: {
       campaignId: 'id',
       couponAmount: 100 
    }
});
```
