LLamaSharp integration

[TesAnti]

implemented IChatModel for LLamaSharp. See LangChain.Providers.LLamaSharp.IntegrationTests for usage examples.

• tests are excluded from CI.
• model is expected to be downloaded manually. working models can be found on LLamaSharp's HugginFace.
• for tests i used model ggml-model-f32-q4_0.bin because it's the smallest.
• i'm using Cpu as a backend in tests so anyone could test it without graphics card and cuda.

to use model in own project - the corresponding backed should be added to the project. This can be done using nuget. For example, LLamaSharp.Backend.Cpu.

closes #32