Support custom request params

[sainnhe]

Issue Description

I'm using this plugin along with qwen3.5, and I want to serve 1 model to handle both fim and chat tasks.

For fim tasks, I want to set repeat-penalty to 1.0, and for chat tasks I want to set it to 1.05. Since the built in web page of llama-server doesn't seem to support custom params, so I need to pass --repeat-penalty to 1.05 to llama-server. But if I do so, I cannot use repeat-penalty=1.0 in llama.vim since this plugin doesn't provide any method to allow users set custom request params.

The Solution

This PR adds 2 config options: fim_params and inst_params. Users can set custom request params via these options. I use extend(..., 'force') so user's config is always prior to the default params.