Data-driven Mob Attributes

Allow us to modify mobs and all their attributes that can be changed using Data packs, WITHOUT relying on execute commands that eats up some performance.

This would also make it much easier to change or add mobs, especially with attributes that requires workarounds like something as simple as Skeleton/Pillager's ranged damage.

I know we've always had ways to change mob stats but it's annoying (and significantly more inefficient) just to change something as simple as number values, it's long overdue.