[DRAFT] OpenXR: Use the XR_FB_foveation_vulkan extension to generate the density map for VRS

[dsnopek]

This builds on top of PR #99551

But instead of manually building the density map, it uses the XR_FB_foveation_vulkan extension to get the density map from the OpenXR runtime.

So, you can use the same foveation settings that are currently used for OpenGL (setting the "Foveation Level" or "Foveation Dynamic"), and that should get used by the OpenXR runtime in order to build the density map that we're using.

I'm not crazy about the interdependencies this PR makes between OpenXRFBFoveationExtension, OpenXRVulkanExtension and OpenXRSwapChainInfo, but I'm not sure there's a way around it? I'll spend some more time thinking about if there's a more decoupled way to do this.

This marked as a DRAFT both because it depends on PR #99551, which isn't merged, but also it's just kind of hacking itself into the renderer. We need some way to use the manually built density map when the XR_FB_foveation_vulkan extension isn't available.