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/iterator_test.go

@@ -170,6 +170,13 @@ func (m *mockProcessor) DecryptBlindedHopData(_ *btcec.PublicKey,
 	return data, m.decryptErr
 }
 
+// NextEphemeral mocks getting our next ephemeral key.
+func (m *mockProcessor) NextEphemeral(*btcec.PublicKey) (*btcec.PublicKey,
+	error) {
+
+	return nil, nil
+}
+
 // TestDecryptAndValidateFwdInfo tests deriving forwarding info using a
 // blinding kit. This test does not cover assertions on the calculations of
 // forwarding information, because this is covered in a test dedicated to those