Support for other ChatGPT Providers

[qwertzui11]

Hi everyone

May I suggest that support/documentation for other ChatGPT-alike providers gets added. eg MistralAi or OpenRouter.

The APIs are very alike and don't need many changes.

In fact OpenRouter needs no changes at all:

      local home = vim.fn.expand("$HOME")
      local model = "mistralai/mistral-medium"
      require("chatgpt").setup({
        api_key_cmd = "cat " .. home .. "/.local/openrouter-apikey",
        api_host_cmd = "echo https://openrouter.ai/api",
        openai_params = {
          model = model,
        },
      })

However MistralAi would need some adoptions:
One has to remove some of the params like top_p, otherwise the MistralAPI will return with the http code 422.

    openai_params = {
      model = "gpt-3.5-turbo",
      frequency_penalty = 0,
      presence_penalty = 0,
      max_tokens = 300,
      temperature = 0,
      top_p = 1,
      n = 1,
    },
    openai_edit_params = {
      model = "gpt-3.5-turbo",
      frequency_penalty = 0,
      presence_penalty = 0,
      temperature = 0,
      top_p = 1,
      n = 1,
    },

However these parameters can't be removed due to M.options = vim.tbl_deep_extend("force", {}, M.defaults(), options)

So I'd like to suggest that openai_params and openai_edit_params might be functions that then create a params list.
Or a setting for a function to build the curl command.
This could also help to resolve the issue #199 with the logging of requests.

What do you think? Shall I provide a Pull Request?

Thx for your time