Simplify getting preimage from send_payment result

[ok300]

To get to the preimage from the payment result of send_payment, the caller has to match the type of payment.details because the PaymentDetails are an enum:

pub enum PaymentDetails {
    Ln {
        #[serde(flatten)]
        data: LnPaymentDetails,
    },
    ClosedChannel {
        #[serde(flatten)]
        data: ClosedChannelPaymentDetails,
    },
}

However we know this could only have been a LnPaymentDetails.

We should simplify this.

Maybe construct a user-facing struct LnPayment, that is identical to Payment except it maps the details to LnPaymentDetails?

[JssDWt]

There is already an issue for this here #302

[ok300]

Duplicate of #302