Include Refund context in blinded payment paths

When requesting a payment for a refund, include a context in the
Bolt12Invoice's blinded payment paths indicated it is for such. When the
eventual payment is received, the user can use the payment hash to
correlate it with the corresponding Refund.

Author: jkczyz

lightning/src/blinded_path/payment.rs

@@ -118,6 +118,10 @@ pub enum PaymentContext {
 		/// [`Offer`]: crate::offers::offer::Offer
 		offer_id: OfferId,
 	},
+	/// The payment was made for invoice sent for a BOLT 12 [`Refund`].
+	///
+	/// [`Refund`]: crate::offers::refund::Refund
+	Bolt12Refund {},
 }
 
 impl TryFrom<CounterpartyForwardingInfo> for PaymentRelay {
@@ -342,6 +346,7 @@ impl_writeable_tlv_based_enum!(PaymentContext,
 	(1, Bolt12Offer) => {
 		(0, offer_id, required),
 	},
+	(2, Bolt12Refund) => {},
 	;
 );
 

lightning/src/ln/channelmanager.rs

@@ -8168,7 +8168,7 @@ where
 
 		match self.create_inbound_payment(Some(amount_msats), relative_expiry, None) {
 			Ok((payment_hash, payment_secret)) => {
-				let payment_context = PaymentContext::Bolt11Invoice {};
+				let payment_context = PaymentContext::Bolt12Refund {};
 				let payment_paths = self.create_blinded_payment_paths(
 					amount_msats, payment_secret, payment_context
 				)