feature,ipn/ipnlocal: add profileManager.StateChangeHook

We update profileManager to allow registering a single state (profile+prefs) change hook.
This is to invert the dependency between the profileManager and the LocalBackend, so that
instead of LocalBackend asking profileManager for the state, we can have profileManager
call LocalBackend when the state changes.

We also update feature.Hook with a new (*feature.Hook).GetOk method to avoid calling both
IsSet and Get.

Updates tailscale/corp#28014
Updates #12614

Signed-off-by: Nick Khyl <nickk@tailscale.com>

feature/feature.go

@@ -53,6 +53,12 @@ func (h *Hook[Func]) Get() Func {
 	return h.f
 }
 
+// GetOk returns the hook function and true if it has been set,
+// otherwise its zero value and false.
+func (h *Hook[Func]) GetOk() (f Func, ok bool) {
+	return h.f, h.ok
+}
+
 // Hooks is a slice of funcs.
 //
 // As opposed to a single Hook, this is meant to be used when