Add pager timer float

I know you don't really care much for stealth, but I really wanted this feature so I figured I'd try and add it myself. I think it'd be nice for others too, so if you want to take this and make it better, I think people will find it useful.

Notes:
* Hooking `CopBrain:clbk_alarm_pager()` was the best I was able to get for detecting when pager timers start, since all my other attempted hooks were failing.
* Setting `self._unit._pager_start_time` was crashing so I had to add it in the unit's interaction.
* The game will crash if the interaction's `_active` is not checked before trying `_alarm_pager_data`.

Bugs:
* Float will disappear before the timer is up.
* Float will stay and continue to count down even after pager is answered or alarm is raised.

Misc:
* Replace magic constant `12` with values from tweak_data (probably try `tweak_data.player.alarm_pager.call_duration`).
* Add a toggle in the options menu for it.