[TF2] Add a config option, so that custom servers can't force you to complete achievements.

[KoxSosen]

In the current system, joining any custom server that doesn't have sv_cheats set to 1 could exploit the system, and give you all of the available achievements. However some players have been collecting them for years, and having them forcibly given by a server unbeknownst to the user is not particularly good.

It would be great if there was an option somewhere is settings, a console command, or something similar where you could disable achievement completion, or progress manually. This would allow players to play on custom servers without worrying about "forced completion".