Create TrampolineForward HTLCSource variant

To process errors returned from downstream nodes when forwarding between
Trampoline nodes, we need to store information beyond what's available
in `HTLCPreviousHopData`, suc as the used hops as the newly generated
outer onion's session_priv.

To that end, we add a new variant to `HTLCSource` in this commit, which
futre-proofs mapping an outbound forward to an incoming MPP.

Co-authored-by: Arik Sosman <[email protected]>

Author: a-mpch

lightning/src/chain/channelmonitor.rs

@@ -2626,6 +2626,7 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 				let outbound_payment = match source {
 					None => panic!("Outbound HTLCs should have a source"),
 					Some(&HTLCSource::PreviousHopData(_)) => false,
+					Some(&HTLCSource::TrampolineForward { .. }) => false,
 					Some(&HTLCSource::OutboundRoute { .. }) => true,
 				};
 				return Some(Balance::MaybeTimeoutClaimableHTLC {
@@ -2835,6 +2836,7 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitor<Signer> {
 					let outbound_payment = match source {
 						None => panic!("Outbound HTLCs should have a source"),
 						Some(HTLCSource::PreviousHopData(_)) => false,
+						Some(HTLCSource::TrampolineForward { .. }) => false,
 						Some(HTLCSource::OutboundRoute { .. }) => true,
 					};
 					if outbound_payment {
@@ -3494,7 +3496,8 @@ impl<Signer: EcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 					}
 				}));
 			}
-			self.counterparty_fulfilled_htlcs.insert(*claimed_htlc_id, *claimed_preimage);
+			let claimed = claimed_htlc_id.clone();
+			self.counterparty_fulfilled_htlcs.insert(claimed, *claimed_preimage);
 		}
 
 		Ok(())