Add cltv expiry to PendingHTLCRouting::Forward

In a coming commit we'll expire HTLCs backwards even if we haven't
yet claimed them on-chain based on their inbound edge being close
to causing a channel force-closure.

Here we track the incoming edge's CLTV expiry in the
pending-routing state so that we can include it in the `HTLCSource`
in the next commit.

Co-authored-by: Matt Corallo <git@bluematt.me>

Author: alecchendev

lightning/src/ln/channelmanager.rs

@@ -169,6 +169,8 @@ pub enum PendingHTLCRouting {
 		short_channel_id: u64, // This should be NonZero<u64> eventually when we bump MSRV
 		/// Set if this HTLC is being forwarded within a blinded path.
 		blinded: Option<BlindedForward>,
+		/// The absolute CLTV of the inbound HTLC
+		incoming_cltv_expiry: Option<u32>,
 	},
 	/// The onion indicates that this is a payment for an invoice (supposedly) generated by us.
 	///
@@ -269,6 +271,14 @@ impl PendingHTLCRouting {
 			_ => None,
 		}
 	}
+
+	fn incoming_cltv_expiry(&self) -> Option<u32> {
+		match self {
+			Self::Forward { incoming_cltv_expiry, .. } => *incoming_cltv_expiry,
+			Self::Receive { incoming_cltv_expiry, .. } => Some(*incoming_cltv_expiry),
+			Self::ReceiveKeysend { incoming_cltv_expiry, .. } => Some(*incoming_cltv_expiry),
+		}
+	}
 }
 
 /// Information about an incoming HTLC, including the [`PendingHTLCRouting`] describing where it
@@ -5522,9 +5532,9 @@ where
 			})?;
 
 		let routing = match payment.forward_info.routing {
-			PendingHTLCRouting::Forward { onion_packet, blinded, .. } => {
+			PendingHTLCRouting::Forward { onion_packet, blinded, incoming_cltv_expiry, .. } => {
 				PendingHTLCRouting::Forward {
-					onion_packet, blinded, short_channel_id: next_hop_scid
+					onion_packet, blinded, incoming_cltv_expiry, short_channel_id: next_hop_scid,
 				}
 			},
 			_ => unreachable!() // Only `PendingHTLCRouting::Forward`s are intercepted
@@ -12433,6 +12443,7 @@ impl_writeable_tlv_based_enum!(PendingHTLCRouting,
 		(0, onion_packet, required),
 		(1, blinded, option),
 		(2, short_channel_id, required),
+		(3, incoming_cltv_expiry, option),
 	},
 	(1, Receive) => {
 		(0, payment_data, required),

lightning/src/ln/onion_payment.rs

@@ -110,6 +110,7 @@ pub(super) fn create_fwd_pending_htlc_info(
 		routing: PendingHTLCRouting::Forward {
 			onion_packet: outgoing_packet,
 			short_channel_id,
+			incoming_cltv_expiry: Some(msg.cltv_expiry),
 			blinded: intro_node_blinding_point.or(msg.blinding_point)
 				.map(|bp| BlindedForward {
 					inbound_blinding_point: bp,