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>