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/hop/forwarding_info.go

@@ -22,4 +22,9 @@ type ForwardingInfo struct {
 	// OutgoingCTLV is the specified value of the CTLV timelock to be used
 	// in the outgoing HTLC.
 	OutgoingCTLV uint32
+
+	// NextBlinding is an optional blinding point to be passed to the next
+	// node in UpdateAddHtlc. This field is set if the htlc is part of a
+	// blinded route.
+	NextBlinding lnwire.BlindingPointRecord
 }