Use [u8; 32] rather than Hmac<Sha256> for simplicity

Mapping an `Hmac<Sha256>` would require somewhat custom logic as
we'd have to behave differently based on generic parameters, so its
simplest to just swap it to a `[u8; 32]` instead.

Author: TheBlueMatt

lightning/src/blinded_path/message.rs

@@ -334,7 +334,7 @@ pub enum OffersContext {
 		/// used with an [`InvoiceError`].
 		///
 		/// [`InvoiceError`]: crate::offers::invoice_error::InvoiceError
-		hmac: Option<Hmac<Sha256>>,
+		hmac: Option<[u8; 32]>,
 	},
 	/// Context used by a [`BlindedMessagePath`] as a reply path for a [`Bolt12Invoice`].
 	///

lightning/src/ln/channelmanager.rs

@@ -9081,7 +9081,7 @@ where
 		let invoice_request = builder.build_and_sign()?;
 
 		let hmac = payment_id.hmac_for_offer_payment(nonce, expanded_key);
-		let context = OffersContext::OutboundPayment { payment_id, nonce, hmac: Some(hmac) };
+		let context = OffersContext::OutboundPayment { payment_id, nonce, hmac: Some(hmac.to_byte_array()) };
 		let reply_paths = self.create_blinded_paths(context)
 			.map_err(|_| Bolt12SemanticError::MissingPaths)?;
 
@@ -10956,7 +10956,7 @@ where
 
 				match context {
 					Some(OffersContext::OutboundPayment { payment_id, nonce, hmac: Some(hmac) }) => {
-						if let Ok(()) = payment_id.verify(hmac, nonce, expanded_key) {
+						if let Ok(()) = payment_id.verify(bitcoin::hashes::hmac::Hmac::from_byte_array(hmac), nonce, expanded_key) {
 							self.abandon_payment_with_reason(
 								payment_id, PaymentFailureReason::InvoiceRequestRejected,
 							);