Expose reply_path from Responder in onion messenger

Add public `reply_path()` method to allow external process or
implementation to access the introduction node information before
responding the onion message.

Author: a-mpch

lightning/src/onion_message/messenger.rs

@@ -413,6 +413,11 @@ impl Responder {
 		Responder { reply_path }
 	}
 
+	/// Returns the reply path for this [`Responder`].
+	pub fn reply_path(&self) -> BlindedMessagePath {
+		self.reply_path.clone()
+	}
+
 	/// Creates a [`ResponseInstruction`] for responding without including a reply path.
 	///
 	/// Use when the recipient doesn't need to send back a reply to us.