Release async payment HTLCs held upstream via OM

If we receive a message that an HTLC is being held upstream for us, send a
reply onion message back releasing it since we are online to receive the
corresponding payment.

lightning/src/ln/channelmanager.rs

@@ -12188,7 +12188,20 @@ where
 		&self, _message: HeldHtlcAvailable, _context: AsyncPaymentsContext,
 		_responder: Option<Responder>
 	) -> Option<(ReleaseHeldHtlc, ResponseInstruction)> {
-		None
+		#[cfg(async_payments)] {
+			match _context {
+				AsyncPaymentsContext::InboundPayment { nonce, hmac, path_absolute_expiry } => {
+					if let Err(()) = signer::verify_held_htlc_available_context(
+						nonce, hmac, &self.inbound_payment_key
+					) { return None }
+					if self.duration_since_epoch() > path_absolute_expiry { return None }
+				},
+				_ => return None
+			}
+			return _responder.map(|responder| (ReleaseHeldHtlc {}, responder.respond()))
+		}
+		#[cfg(not(async_payments))]
+		return None
 	}
 
 	fn handle_release_held_htlc(&self, _message: ReleaseHeldHtlc, _context: AsyncPaymentsContext) {

lightning/src/offers/signer.rs

@@ -501,3 +501,10 @@ pub(crate) fn hmac_for_held_htlc_available_context(
 
 	Hmac::from_engine(hmac)
 }
+
+#[cfg(async_payments)]
+pub(crate) fn verify_held_htlc_available_context(
+	nonce: Nonce, hmac: Hmac<Sha256>, expanded_key: &ExpandedKey,
+) -> Result<(), ()> {
+	if hmac_for_held_htlc_available_context(nonce, expanded_key) == hmac { Ok(()) } else { Err(()) }
+}