Make OnionMessageHandler indicate whether forwarding is supported

Used in upcoming commit(s) to decide whether to advertise onion message support
in features.

Author: valentinewallace

lightning/src/ln/msgs.rs

@@ -955,6 +955,8 @@ pub trait OnionMessageHandler : OnionMessageProvider {
 	/// Indicates a connection to the peer failed/an existing connection was lost. Allows handlers to
 	/// drop and refuse to forward onion messages to this peer.
 	fn peer_disconnected(&self, their_node_id: &PublicKey, no_connection_possible: bool);
+	/// Whether this object supports forwarding onion messages.
+	fn supports_onion_message_forwarding(&self) -> bool;
 }
 
 mod fuzzy_internal_msgs {

lightning/src/ln/peer_handler.rs

@@ -83,6 +83,7 @@ impl OnionMessageHandler for IgnoringMessageHandler {
 	fn handle_onion_message(&self, _their_node_id: &PublicKey, _msg: &msgs::OnionMessage) {}
 	fn peer_connected(&self, _their_node_id: &PublicKey, _init: &msgs::Init) {}
 	fn peer_disconnected(&self, _their_node_id: &PublicKey, _no_connection_possible: bool) {}
+	fn supports_onion_message_forwarding(&self) -> bool { false }
 }
 impl Deref for IgnoringMessageHandler {
 	type Target = IgnoringMessageHandler;

lightning/src/onion_message/messenger.rs

@@ -318,6 +318,8 @@ impl<Signer: Sign, K: Deref, L: Deref> OnionMessageHandler for OnionMessenger<Si
 		let mut pending_msgs = self.pending_messages.lock().unwrap();
 		pending_msgs.remove(their_node_id);
 	}
+
+	fn supports_onion_message_forwarding(&self) -> bool { true }
 }
 
 impl<Signer: Sign, K: Deref, L: Deref> OnionMessageProvider for OnionMessenger<Signer, K, L>