Blink plugin: BTCPay 2.0.0 incompatibility

[ndeet]

see btcpayserver/btcpayserver#6200 the issue seems to come from blink plugin and

[NicolasDorier]

This will be fixed by btcpayserver/btcpayserver#6202
Basically BTCPay 2.0, use have msat precision for LN, but Blink don't support it. The issue of providers not creating exactly the amount expect isn't limited to Blink, @rockstardev also had the problem with Strike.

As such, I came up with a "hack" where I add a positive or negative paymentMethodFee (I call that a tweakFee) when that happen, that way the amount returned by the lightning client is matching the amount of invoice + fee, and there is no under/over payments.

This fee won't show up in the checkout but nevertheless exists.