htlcswitch: add InterceptableSwitchConfig

Preparation for adding more config options.

server.go

@@ -667,8 +667,11 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		return nil, err
 	}
 	s.interceptableSwitch = htlcswitch.NewInterceptableSwitch(
-		s.htlcSwitch, lncfg.DefaultFinalCltvRejectDelta,
-		s.cfg.RequireInterceptor,
+		&htlcswitch.InterceptableSwitchConfig{
+			Switch:             s.htlcSwitch,
+			CltvRejectDelta:    lncfg.DefaultFinalCltvRejectDelta,
+			RequireInterceptor: s.cfg.RequireInterceptor,
+		},
 	)
 
 	s.witnessBeacon = newPreimageBeacon(