control/controlclient: introduce eventbus messages instead of callbacks (#16956)

This is a small introduction of the eventbus into controlclient that
communicates with mainly ipnlocal. While ipnlocal is a complicated part
of the codebase, the subscribers here are from the perspective of
ipnlocal already called async.

Updates #15160

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

util/eventbus/eventbustest/eventbustest.go

@@ -15,7 +15,7 @@ import (
 
 // NewBus constructs an [eventbus.Bus] that will be shut automatically when
 // its controlling test ends.
-func NewBus(t *testing.T) *eventbus.Bus {
+func NewBus(t testing.TB) *eventbus.Bus {
 	bus := eventbus.New()
 	t.Cleanup(bus.Close)
 	return bus