Add support for fulfillment payload #3321
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We refactor the attribution data code to make it more consistent with the rest of the Sphinx-related code. We add comments and intermediate variables to make it more readable. We introduce intermediate classes to hold data and add better symmetry between the success and failure cases. This will make it easier to implement trampoline attribution and add fulfillment data (lightning/bolts#1344).
We correctly extract shared secrets (including trampoline shared secrets) and detect when we're inside a blinded path to avoid including attribution data. Note that we don't use the trampoline shared secret yet, since full trampoline isn't implemented (see #2819 for the full changes). We add tests for attribution data with blinded paths and trampoline payments.
This adds support for including a payload in `update_fulfill_htlc` that the recipient encrypts for the payer. This can be useful to transmit data atomically with the fulfillment of a payment. The main challenge is that intermediate nodes may drop or tamper with this fulfillment payload, which is why we include it in the HMACs of the attribution data, which lets senders detect which pair of nodes may be malicious. See lightning/bolts#1344
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This adds support for including a payload in
update_fulfill_htlcthat the recipient encrypts for the payer. This can be useful to transmit data atomically with the fulfillment of a payment. The main challenge is that intermediate nodes may drop or tamper with this fulfillment payload, which is why we include it in the HMACs of the attribution data, which lets senders detect which pair of nodes may be malicious.See lightning/bolts#1344
Builds on top of #3320
TODO: