ShaderGraph BoneIndex & BoneWeight Nodes

[zezba9000]

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Purpose of this PR

Missing ShaderGraph nodes for doing custom GPU Vertex skinning shaders.

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Testing status

Wrote custom URP ShaderGraph shader that does GPU Vertex Skinning to verify function.

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Comments to reviewers

This is highly useful for Quest / VR situation where Unity's build in SkinnedMeshRenderer can cause notable performance degradation. Exposing BoneIndexNode & BoneWeightNode in URP ShaderGraph allow for special / platform optimized skinned shaders to be made.