Feature request: xpay should allow overpayment

I want to be able to overpay an invoice using xpay.
This is a common method to tip the receiver.

According to the documentation of `xpay` I cannot specify an amount if already get an invoice.

```
**amount_msat** (msat, optional): Only possible for a bolt11 invoice which does not have an amount (in which case, it's compulsory). amount_msat is in millisatoshi precision; it can be a whole number, or a whole number with suffix msat or sat, or a three decimal point number with suffix sat, or an 1 to 11 decimal point number suffixed by btc.
```

I want to be able to overpay.
If I specify an amount which is equal or greater (upto a safety limit) then the invoice amount the payment should succeed.