GDK CMake support (Xbox Series X only for now)

[Arcnor]

Description

This adds support to build Xbox Series X using CMake. It's a bit of a mess, hence marking it as a draft. This can obviously be updated so support is added to Xbox One & Desktop too, but I haven't had the time to test those yet.

Because there is no "official" CMake support, I've used GDK_GAMING_XBOX and friends to detect support for it. I was convinced I got those from the official CMake examples here https://github.com/microsoft/Xbox-GDK-Samples/blob/main/Samples/Tools/CMakeExample/CMake/CMakeGDKScarlett.cmake but taking a look at them now I can't find that definition (my defs are many years old at this point, maybe this simply changed), so this should probably be changed, or some kind of convention added to the docs for CMake support.

Really open to suggestions before moving to "normal" PR status.