Add a MessageSendInstructions::ForReply

In order to allow onion message handlers to reply asynchronously
without introducing a circular dependency graph, the message
handlers need to be able to send replies described by
`MessageSendInstructions`. This allows them to send replies via the
normal message queuing (i.e. without making a function call to
`OnionMessenger`).

Here we enable that by adding a `MessageSendInstructions::ForReply`
variant which holds `ReplyInstruction`s.

Fixes #3178

lightning/src/onion_message/messenger.rs

@@ -367,7 +367,7 @@ impl Responder {
 	/// Use when the recipient doesn't need to send back a reply to us.
 	pub fn respond(self) -> ResponseInstruction {
 		ResponseInstruction {
-			send_path: self.reply_path,
+			destination: Destination::BlindedPath(self.reply_path),
 			context: None,
 		}
 	}
@@ -377,7 +377,7 @@ impl Responder {
 	/// Use when the recipient needs to send back a reply to us.
 	pub fn respond_with_reply_path(self, context: MessageContext) -> ResponseInstruction {
 		ResponseInstruction {
-			send_path: self.reply_path,
+			destination: Destination::BlindedPath(self.reply_path),
 			context: Some(context),
 		}
 	}
@@ -386,17 +386,13 @@ impl Responder {
 /// Instructions for how and where to send the response to an onion message.
 #[derive(Clone)]
 pub struct ResponseInstruction {
-	send_path: BlindedMessagePath,
+	destination: Destination,
 	context: Option<MessageContext>,
 }
 
 impl ResponseInstruction {
 	fn into_instructions(self) -> MessageSendInstructions {
-		let destination = Destination::BlindedPath(self.send_path);
-		match self.context {
-			Some(context) => MessageSendInstructions::WithReplyPath { destination, context },
-			None => MessageSendInstructions::WithoutReplyPath { destination },
-		}
+		MessageSendInstructions::ForReply { instructions: self }
 	}
 }
 
@@ -425,7 +421,11 @@ pub enum MessageSendInstructions {
 	WithoutReplyPath {
 		/// The desination where we need to send our message.
 		destination: Destination,
-	}
+	},
+	/// Indicates that a message is being sent as a reply to a received message.
+	ForReply {
+		instructions: ResponseInstruction,
+	},
 }
 
 /// A trait defining behavior for routing an [`OnionMessage`].
@@ -1167,9 +1167,11 @@ where
 		&self, message: T, instructions: MessageSendInstructions, log_suffix: fmt::Arguments,
 	) -> Result<SendSuccess, SendError> {
 		let (destination, reply_path) = match instructions {
-			MessageSendInstructions::WithSpecifiedReplyPath { destination, reply_path } =>
+			MessageSendInstructions::WithSpecifiedReplyPath { destination, reply_path }|
+			MessageSendInstructions::ForReply { instructions: ResponseInstruction { context: None, destination } } =>
 				(destination, Some(reply_path)),
-			MessageSendInstructions::WithReplyPath { destination, context } => {
+			MessageSendInstructions::WithReplyPath { destination, context }|
+			MessageSendInstructions::ForReply { instructions: ResponseInstruction { destination, context } } => {
 				match self.create_blinded_path(context) {
 					Ok(reply_path) => (destination, Some(reply_path)),
 					Err(err) => {