Add managed wrapper for D3DX12_MESH_SHADER_PIPELINE_STATE_DESC

[Jorgemagic]

I’m working on a DirectX 12 mesh‑shader sample using Vortice, and I noticed that there isn’t a high‑level .NET struct corresponding to the native D3DX12_MESH_SHADER_PIPELINE_STATE_DESC. As a result, I have to manually build a raw byte‑stream via PipelineStateStreamDescription and pin all subobjects (root signature, AS, MS, PS, blend, rasterizer, etc.), which is error‑prone and verbose.

It would be really helpful if Vortice exposed a dedicated MeshShaderPipelineStateDescription (or similar) that mirrors the native helper struct. That way, users could fill out its fields in a type‑safe manner—just like GraphicsPipelineStateDescription—and call CreatePipelineState without working directly with pointers and Marshal.StructureToPtr.

Reference:
DX12 official specification

The PipelineStateSubObjectType enum already includes AmplificationShader and MeshShader, so adding this wrapper would complete the mesh‑shader pipeline‑state API in Vortice. Thanks!