Completion model provider should be traitlet-configurable

[krassowski]

@krassowski Will it also be better if we can set something like default_completion_model and default_chat_model when running jupyter lab --config using config.json? Previously, we can only do something like:

{
    "AiExtension": {
        "default_language_model": "Self-Hosted:deepseek-coder-6.7b-instruct",
        "allowed_providers": "Self-Hosted",
        "model_parameters": {
            "Self-Hosted:deepseek-coder-6.7b-base": {},
            "Self-Hosted:deepseek-coder-6.7b-instruct": {}
        },
        "default_api_keys":{}
    }
}

It may be better to modify the extension.py also. Thank you.

Originally posted by @yuhon0528 in https://github.com/jupyterlab/jupyter-ai/issues/711#issuecomment-2104050127