Provide API to quickly check XeFG/XeLL device support

Hi,

Currently the only way to determine whether a device supports XeLL and XeFG seems to be by creating a context (`xellD3D12CreateContext` and `xefgSwapChainD3D12CreateContext`) and checking the return value.
This is slow and very inconvenient because we simply want to check support on startup without already allocating resources.
Is there an API to quickly check for feature support, rather than creating a context, which has an unacceptable overhead?

Thanks in advance,
Simon