Split platform-specific binaries for NuGet backends

[m0nsky]

This PR refactors the NuGet package structure for the LLamaSharp.Backend.Cuda11, LLamaSharp.Backend.Cuda12 and LLamaSharp.Backend.Vulkan packages by splitting the platform-specific binaries (Windows and Linux) into separate packages, and adds them as dependencies on the main packages. This way we can work around the 250MB limit for NuGet which broke the latest release.

Package Name	File Size Before (MB)	File Size After (MB)
LLamaSharp.Backend.Cuda11	286.36 MB	0.032 MB
LLamaSharp.Backend.Cuda11.Windows	N/A	142.56 MB
LLamaSharp.Backend.Cuda11.Linux	N/A	143.82 MB
LLamaSharp.Backend.Cuda12	284.33 MB	0.032 MB
LLamaSharp.Backend.Cuda12.Windows	N/A	141.62 MB
LLamaSharp.Backend.Cuda12.Linux	N/A	142.74 MB
LLamaSharp.Backend.Vulkan	3.49 MB	0.032 MB
LLamaSharp.Backend.Vulkan.Windows	N/A	1.54 MB
LLamaSharp.Backend.Vulkan.Linux	N/A	1.98 MB

[m0nsky]

I tested installing the CUDA & Vulkan packages today and it seems to be working fine.