multi: explicitly signal final hop in pack hop payload

Previously, we'd use the value of nextChanID to infer whether a payload
was for the final hop in a route. This commit updates our packing logic
to explicitly signal to account for blinded routes, which allow zero
value nextChanID in intermediate hops. This is a preparatory commit
that allows us to more thoroughly validate payloads.

htlcswitch/hop/fuzz_test.go

@@ -121,7 +121,7 @@ func fuzzPayload(f *testing.F, finalPayload bool) {
 
 		var b bytes.Buffer
 		hop, nextChanID := hopFromPayload(payload1)
-		err = hop.PackHopPayload(&b, nextChanID)
+		err = hop.PackHopPayload(&b, nextChanID, finalPayload)
 		if errors.Is(err, route.ErrAMPMissingMPP) {
 			// PackHopPayload refuses to encode an AMP record
 			// without an MPP record. However, NewPayloadFromReader