Manually port d3dx12.h as expansions partials to generated code

[d3dx12.h](https://github.com/microsoft/DirectX-Headers/blob/main/include/directx/d3dx12.h) contains really useful helper APIs for working with Direct3D 12. We should consider porting these to Silk.NET.Direct3D12 to make life easier for our users.

This isn't a hard task, just involves creating additional, manual partials for each struct d3dx12 has helper methods for, or methods in the manual part of the D3D12 class. As such, any help from the community would be appreciated.