Base implementation of /datum/persistent_client (#89449)

## About The Pull Request
Converts `/datum/player_details` into `/datum/persistent_client`.
Persistent Clients persist across connections. The only time a mob's
persistent client will change is if the ckey it's bound to logs into a
different mob, or the mob is deleted (duh).

Also adds PossessByPlayer() so that transfering mob control is cleaner
and makes more immediate sense if you don't know byond-fu.

## Why It's Good For The Game
Clients are an abstract representation of a connection that can be
dropped at almost any moment so putting things that should be stable to
access at any time onto an undying object is ideal. This allows for
future expansions like abstracting away client.screen and managing
everything cleanly.

code/modules/mob/dead/observer/observer.dm

@@ -283,7 +283,7 @@ Works together with spawning an observer, noted above.
 	var/mob/dead/observer/ghost = new(src) // Transfer safety to observer spawning proc.
 	SStgui.on_transfer(src, ghost) // Transfer NanoUIs.
 	ghost.can_reenter_corpse = can_reenter_corpse
-	ghost.key = key
+	ghost.PossessByPlayer(key)
 	ghost.client?.init_verbs()
 	if(!can_reenter_corpse)// Disassociates observer mind from the body mind
 		ghost.mind = null
@@ -293,7 +293,7 @@ Works together with spawning an observer, noted above.
 	if(isliving(former_mob))
 		recordable_time = former_mob.timeofdeath
 
-	ghost.client?.player_details.time_of_death = recordable_time
+	ghost.persistent_client?.time_of_death = recordable_time
 	SEND_SIGNAL(src, COMSIG_MOB_GHOSTIZED)
 	return ghost
 
@@ -386,7 +386,7 @@ This is the proc mobs get to turn into a ghost. Forked from ghostize due to comp
 	SStgui.on_transfer(src, mind.current) // Transfer NanoUIs.
 	if(mind.current.stat == DEAD && SSlag_switch.measures[DISABLE_DEAD_KEYLOOP])
 		to_chat(src, span_warning("To leave your body again use the Ghost verb."))
-	mind.current.key = key
+	mind.current.PossessByPlayer(key)
 	mind.current.client.init_verbs()
 	return TRUE
 
@@ -680,7 +680,7 @@ This is the proc mobs get to turn into a ghost. Forked from ghostize due to comp
 		to_chat(src, span_warning("Someone has taken this body while you were choosing!"))
 		return FALSE
 
-	target.key = key
+	target.PossessByPlayer(key)
 	target.faction = list(FACTION_NEUTRAL)
 	return TRUE