Fireable Zombie spawn entity

[F0mx]

Either a spawn flag for the spawn_zombie entity or a new one that does not follow typical spawning mechanics, instead spawning a single zombie when fired at by say, a game_counter.

Would really open up some options for mapping, off the top of my head it'd allow both the dig site zombie spawns from origins, the pre-teleport infinite wave of moon, and in general non-round based maps with a little bit of creativity with logic entities.