htlcswitch: add NextBlinding to ForwardingInfo and set in UpdateAddHtlc

When we have a HTLC that is part of a blinded route, we need to include
the next ephemeral blinding point in UpdateAddHtlc for the next hop. The
way that we handle the addition of this key is the same for introduction
nodes and relaying nodes within the route.

htlcswitch/link.go

@@ -3348,9 +3348,10 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 				// Otherwise, it was already processed, we can
 				// can collect it and continue.
 				addMsg := &lnwire.UpdateAddHTLC{
-					Expiry:      fwdInfo.OutgoingCTLV,
-					Amount:      fwdInfo.AmountToForward,
-					PaymentHash: pd.RHash,
+					Expiry:        fwdInfo.OutgoingCTLV,
+					Amount:        fwdInfo.AmountToForward,
+					PaymentHash:   pd.RHash,
+					BlindingPoint: fwdInfo.NextBlinding,
 				}
 
 				// Finally, we'll encode the onion packet for
@@ -3393,9 +3394,10 @@ func (l *channelLink) processRemoteAdds(fwdPkg *channeldb.FwdPkg,
 			// create the outgoing HTLC using the parameters as
 			// specified in the forwarding info.
 			addMsg := &lnwire.UpdateAddHTLC{
-				Expiry:      fwdInfo.OutgoingCTLV,
-				Amount:      fwdInfo.AmountToForward,
-				PaymentHash: pd.RHash,
+				Expiry:        fwdInfo.OutgoingCTLV,
+				Amount:        fwdInfo.AmountToForward,
+				PaymentHash:   pd.RHash,
+				BlindingPoint: fwdInfo.NextBlinding,
 			}
 
 			// Finally, we'll encode the onion packet for the