f allow sending any custom OM, not just those understood by the handler

TODO do we want to allow this? it makes the call signature nicer and adds flexibility

Author: valentinewallace

lightning/src/onion_message/messenger.rs

@@ -167,7 +167,7 @@ impl<Signer: Sign, K: Deref, L: Deref, CMH: Deref> OnionMessenger<Signer, K, L,
 
 	/// Send an onion message with contents `message` to `destination`, routing it through `intermediate_nodes`.
 	/// See [`OnionMessenger`] for example usage.
-	pub fn send_onion_message(&self, intermediate_nodes: &[PublicKey], destination: Destination, message: Message<<<CMH as Deref>::Target as CustomOnionMessageHandler>::CustomMessage>, reply_path: Option<BlindedRoute>) -> Result<(), SendError> {
+	pub fn send_onion_message<T: OnionMessageContents>(&self, intermediate_nodes: &[PublicKey], destination: Destination, message: Message<T>, reply_path: Option<BlindedRoute>) -> Result<(), SendError> {
 		if let Destination::BlindedRoute(BlindedRoute { ref blinded_hops, .. }) = destination {
 			if blinded_hops.len() < 2 {
 				return Err(SendError::TooFewBlindedHops);