Add Event::ConnectionNeeded for onion messages

A MessageRouter may be unable to find a complete path to an onion
message's destination. This could because no such path exists or any
needs on a potential path don't support onion messages. Add an event
that indicates a connection with a node is needed in order to send the
message.

Author: jkczyz

lightning/src/events/mod.rs

@@ -513,6 +513,23 @@ pub enum Event {
 		/// serialized prior to LDK version 0.0.117.
 		sender_intended_total_msat: Option<u64>,
 	},
+	/// Indicates that a peer connection with a node is needed in order to send an [`OnionMessage`].
+	///
+	/// Typically, this happens when a [`MessageRouter`] is unable to find a complete path to a
+	/// [`Destination`]. Once a connection is established, any messages buffered by an
+	/// [`OnionMessageHandler`] may be sent.
+	///
+	/// This event will not be generated for onion message forwards; only for sends including
+	/// replies. Handlers should connect to the node otherwise any buffered messages may be lost.
+	///
+	/// [`OnionMessage`]: msgs::OnionMessage
+	/// [`MessageRouter`]: crate::onion_message::MessageRouter
+	/// [`Destination`]: crate::onion_message::Destination
+	/// [`OnionMessageHandler`]: crate::ln::msgs::OnionMessageHandler
+	ConnectionNeeded {
+		/// Information needed to decide on whether to and how to connect to the node.
+		node_announcement: msgs::NodeAnnouncement,
+	},
 	/// Indicates a request for an invoice failed to yield a response in a reasonable amount of time
 	/// or was explicitly abandoned by [`ChannelManager::abandon_payment`]. This may be for an
 	/// [`InvoiceRequest`] sent for an [`Offer`] or for a [`Refund`] that hasn't been redeemed.
@@ -1173,6 +1190,12 @@ impl Writeable for Event {
 					(0, payment_id, required),
 				})
 			},
+			&Event::ConnectionNeeded { ref node_announcement } => {
+				35u8.write(writer)?;
+				write_tlv_fields!(writer, {
+					(0, node_announcement, required),
+				})
+			},
 			// Note that, going forward, all new events must only write data inside of
 			// `write_tlv_fields`. Versions 0.0.101+ will ignore odd-numbered events that write
 			// data via `write_tlv_fields`.
@@ -1571,6 +1594,17 @@ impl MaybeReadable for Event {
 				};
 				f()
 			},
+			35u8 => {
+				let f = || {
+					_init_and_read_len_prefixed_tlv_fields!(reader, {
+						(0, node_announcement, required),
+					});
+					Ok(Some(Event::ConnectionNeeded {
+						node_announcement: node_announcement.0.unwrap(),
+					}))
+				};
+				f()
+			},
 			// Versions prior to 0.0.100 did not ignore odd types, instead returning InvalidValue.
 			// Version 0.0.100 failed to properly ignore odd types, possibly resulting in corrupt
 			// reads.