feat: multiple models

[vansangpfiev]

Issue link: https://github.com/janhq/cortex/issues/467

• use std::unordered_map to store all llama_server_context (lsc)
• refactor: move background thread to lsc, each lsc has its own background thread

We use mode_id as a key to find the model, so they have to be unique. That requires some changes in request parameter:
For completions:
Have to set value the model_alias for inferences/llamaCPP/loadmodel and inferences/llamaCPP/unloadmodel , this has to be the same as model parameter in inferences/llamaCPP/chat_completion
loadmodel/unloadmodel:

{
...
 "llama_model_path": "file_to_location"
  "model_alias": "model1"
...
}

chat_completion:

{
...
  "model": "model1"  
...
}

For embeddings:
The same for loadmodel/unloadmodel, for embedding request, we need to add model parameter
loadmodel/unloadmodel

{
...
    "llama_model_path": "e:/workspace/model/nomic-embed-text-v1.5.f16.gguf",
    "model_alias": "model1",
    "model_type": "embedding"
...
}

embedding

{
...  
  "input": "how are you",
  "model": "model1"
...
}

[tikikun]

LGTM!!!