Introduce TrampolineHop struct

To define the hops in a Path that are meant as Trampoline hops, we
need to introduce a new hop type much alike `RouteHop`, except these
hops would not be defined by channel ID used to route payments.

Instead, the preceding Trampoline hop would determine its own optimal
path to route the payment onwards.

Author: arik-so

lightning/src/routing/router.rs

@@ -429,6 +429,31 @@ impl_writeable_tlv_based!(RouteHop, {
 	(10, cltv_expiry_delta, required),
 });
 
+/// A Trampoline hop in a route, and additional metadata about it. "Hop" is defined as a node.
+#[derive(Clone, Debug, Hash, PartialEq, Eq)]
+pub struct TrampolineHop {
+	/// The node_id of the node at this hop.
+	pub pubkey: PublicKey,
+	/// The node_announcement features of the node at this hop.
+	pub node_features: NodeFeatures,
+	/// The fee this hop should use to pay for routing towards the next Trampoline hop, or to the
+	/// recipient if this is the last Trampoline hop.
+	/// If this is the last Trampoline hop within [`Path`], this is the fee paid for the use of
+	/// the entire blinded path, if one exists.
+	pub fee_msat: u64,
+	/// The CLTV delta added for this hop.
+	/// If this is the last Trampoline hop within [`Path`], this is the CLTV delta for the entire
+	/// blinded path, if one exists.
+	pub cltv_expiry_delta: u32,
+}
+
+impl_writeable_tlv_based!(TrampolineHop, {
+	(0, pubkey, required),
+	(2, node_features, required),
+	(4, fee_msat, required),
+	(6, cltv_expiry_delta, required),
+});
+
 /// The blinded portion of a [`Path`], if we're routing to a recipient who provided blinded paths in
 /// their [`Bolt12Invoice`].
 ///