Make FinalOnionHopData public

In 4b5db8c, `channelmanager::PendingHTLCRouting` was made
public, exposing a `FinalOnionHopData` field to the world. However,
`FinalOnionHopData` was left crate-private, making the enum
impossible to construct.

There isn't a strong reason for this (even though the
`FinalOnionHopData` API is somewhat confusing, being separated from
the rest of the onion structs), so we expose it here.

lightning/src/ln/msgs.rs

@@ -1668,23 +1668,31 @@ pub trait OnionMessageHandler: EventsProvider {
 	fn provided_init_features(&self, their_node_id: &PublicKey) -> InitFeatures;
 }
 
+#[derive(Clone)]
+#[cfg_attr(test, derive(Debug, PartialEq))]
+/// Information communicated in the onion to the recipient for multi-part tracking and proof that
+/// the payment is associated with an invoice.
+pub struct FinalOnionHopData {
+	/// When sending a multi-part payment, this secret is used to identify a payment across HTLCs.
+	/// Because it is generated by the recipient and included in the invoice, it also provides
+	/// proof to the recipient that the payment was sent by someone with the generated invoice.
+	pub payment_secret: PaymentSecret,
+	/// The intended total amount that this payment is for.
+	///
+	/// Message serialization may panic if this value is more than 21 million Bitcoin.
+	pub total_msat: u64,
+}
+
 mod fuzzy_internal_msgs {
 	use bitcoin::secp256k1::PublicKey;
 	use crate::blinded_path::payment::{PaymentConstraints, PaymentRelay};
 	use crate::prelude::*;
 	use crate::ln::{PaymentPreimage, PaymentSecret};
 	use crate::ln::features::BlindedHopFeatures;
+	use super::FinalOnionHopData;
 
 	// These types aren't intended to be pub, but are exposed for direct fuzzing (as we deserialize
 	// them from untrusted input):
-	#[derive(Clone)]
-	#[cfg_attr(test, derive(Debug, PartialEq))]
-	pub struct FinalOnionHopData {
-		pub payment_secret: PaymentSecret,
-		/// The total value, in msat, of the payment as received by the ultimate recipient.
-		/// Message serialization may panic if this value is more than 21 million Bitcoin.
-		pub total_msat: u64,
-	}
 
 	pub enum InboundOnionPayload {
 		Forward {