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

@@ -10893,7 +10893,12 @@ where
 				};
 
 				match response {
-					Ok(invoice) => Some((OffersMessage::Invoice(invoice), responder.respond())),
+					Ok(invoice) => {
+						let nonce = nonce.unwrap_or_else(|| Nonce::from_entropy_source(&*self.entropy_source));
+						let hmac = signer::hmac_for_payment_hash(payment_hash, nonce, expanded_key);
+						let context = MessageContext::Offers(OffersContext::InboundPayment { payment_hash, nonce, hmac });
+						Some((OffersMessage::Invoice(invoice), responder.respond_with_reply_path(context)))
+					},
 					Err(error) => Some((OffersMessage::InvoiceError(error.into()), responder.respond())),
 				}
 			},