Steamworks API integration

[sr229]

As part of #251, we might want to include Steamworks API just in case people might have a use for it on our Steam releases. It would be optional and will be hidden under a build flag.

[sr229]

We'll use Facepunch's API for this, the problem is idk if they have a NuGet package for this.

[LeNitrous]

https://www.nuget.org/packages/Facepunch.Steamworks/