CLN Hold Invoice Plugin: Handle Overpayments

[kilrau]

Currently we are processing swaps as usual when users overpay and there is no limit on how much they can overpay. Onchain (Normal Swaps) there is nothing we can do and rely on the user contacting us with swap details for a refund but for Lightning (Reverse Swaps) we should adhere to BOLT04 behavior:

if the amount paid is more than twice the amount expected:
SHOULD fail the HTLC.
SHOULD return an incorrect_or_unknown_payment_details error.
Note: this allows the origin node to reduce information leakage by altering the amount while not allowing for accidental gross overpayment.

• immediately fail incoming payments for a hold invoice that already has a pending payment and
• immediately fail payments that overpay 2x or more as per above BOLT04 definition.

LND's integrated holdinvoice implemenation already behaves like this. Our own CLN hold invoice plugin should too.