[Feat] config for model selection for both local and other providers

[PyDevC]

Files: lua/model_cmp/config.lua:14-32, gemini.lua:58, llama.lua:38

Feature Introduction: Models are hardcoded ("gemini-2.0-flash", "llama"). Users cannot choose which model variant to use.

Feature Description:

require("model_cmp").setup({
    api = {
        model = "gemini-2.5-pro",  -- or "codestral-latest", "deepseek-coder"
    }
})

Pass the model name to each provider's generate_request() function.

Is your feature possible with the current status of repo? Yes. Requires adding a model field to config and passing it through the request pipeline.