[0045] Clustered Geometry: Add ALLOW_CLUSTERED_GEOMETRY flag to raytracing pipeline state and RAYQUERY_FLAGS

[amarpMSFT]

Pasting relevant text from motivation section in the spec:

There also needs to be a way for apps to tell driver shader compilation to expect that
acceleration structures being raytraced may contain clusters. In the API this is a
raytracing pipeline flag, D3D12_RAYTRACING_PIPELINE_FLAG_ALLOW_CLUSTERED_GEOMETRY.
This flag needs to be available when raytracing pipeline state subobjects are
authored in HLSL, including RayQuery objects for inline raytracing.

The reason for the flag is if possibility of clusters in an acceleration structure requires
a different path in the driver/hardware ray traversal algorithm. This path might add
overhead that can be avoided if it is known clusters will not be used. In other words,
a driver that supports clustered geometry should not regress the performance of applications
that choose not to use it, or shipped before clustered geometry existed.

This addition builds on similar HLSL/DXIL functionality that was needed for Opacity Micromaps. See https://github.com/microsoft/hlsl-specs/blob/main/proposals/0024-opacity-micromaps.md.
So this time around it is easier to make the addition.