Add reply_path to BOLT12Invoices in Offers Flow

1. Introduced reply_path in BOLT12Invoices to address a gap in error handling.
   Previously, if a BOLT12Invoice sent in the offers flow generated an Invoice Error,
   the payer had no way to send this error back to the payee.
2. By adding a reply_path to the Invoice Message, the payer can now communicate
   any errors back to the payee, ensuring better error handling and communication
   within the offers flow.

Author: shaavan

lightning/src/ln/channelmanager.rs

@@ -10620,7 +10620,10 @@ where
 				};
 
 				match response {
-					Ok(invoice) => responder.respond(OffersMessage::Invoice(invoice)),
+					Ok(invoice) => {
+						let context = MessageContext::Offers(OffersContext::Unknown {});
+						responder.respond_with_reply_path(OffersMessage::Invoice(invoice), context)
+					},
 					Err(error) => responder.respond(OffersMessage::InvoiceError(error.into())),
 				}
 			},