Support turning off the perf monitor

[swz-git]

There is already the https://github.com/RLBot/core/tree/perfmonitor_env_override branch, but it uses an env var. I think this should be done through the schema instead.

[NicEastvillage]

I agree. There should probably be 3 possible values: AlwaysShow, NeverShow, and ShowWhenSupoptimal.