Add HighPerformanceSession and use LowLatency GC in gameplay

[smoogipoo]

Slightly adapted from #11996. Exposes virtual methods for the PR to eventually override, and to potentially support different "high performance" profiles on mobile devices.

In debugging GC stutters, I noticed that using LowLatency GC mode yields better frame consistency in cases where we're not allocating very much (i.e. basically only in gameplay). This is also what osu!stable does, however it also allocates 10x less than we currently are - a good goal to aim for in any case.
Here's some graphs to look at: https://docs.google.com/spreadsheets/d/1ZJLe61ay_O1JT1EbpphQcGpKdaoYro2jR_WOkecfyB8/edit#gid=0

I'm still following up with the .NET team in dotnet/runtime#48937, but this should serve as a good test for the time being.

Only has an effect if the local user is playing. Has no effect in replays.

[peppy]

Guessing if this sticks we may want to enable for replays / spectator as well. Although that's less of a fine-cut decision as the local user may be interacting with overlays, which could end up degrading performance?

[smoogipoo]

If overlays allocate a lot yeah, but I don't disagree in principle. I hope that this is a temporary thing.

[bdach]

Curious what the general response to this will be. Hopefully positive.

👍