Use a weighted pool system instead of hardcoded chances for each role

[cg-223]

There are a few problems this solves.
One of them is that custom roles are chosen independently of actual player roles, which leads to needing to do weird things like reducing spawn rates when the player count is low (which has many problems, like just flat out making the SCP team much less likely to have custom roles).

It also would fix the issue with high spawn rate boosts leading to the exact same role being picked every time.

I also fix waves having the same roles in them every time with this.

Also, some things would be fixed that aren't very noticeable issues (if a godlike custom role spawns but no player is available to be replaced by that godlike, nothing happens. this wouldn't matter, but if this happens in a game it would mean that no other godlike role would ever even get a chance to roll to spawn).

I believe this approach has better performance as well, although I can't confirm this with benchmarks.

This would also be breaking, all roles have a different chance of spawning now (mostly because their spawn chances can't overlap anymore). Some adjustments might have to be made to individual role chances to make them feel the same as before.

Several new configs are added with this, to hopefully make fiddling with odds easier. It should be possible to do more things with modifying chances in SCR by simply editing the main config file (or the group config file) rather than every individual custom role file.

[cg-223]

@SlejmUr don't have perms to add you as reviewer, but should be ready now