Allow OpenAI endpoint configuration via config

[k2589]

When a user wants to use other API-compatible services or use a local instance of GPT, might be needed to redefine the OpenAI base URL.

With new configuration option openai_base_url, the user is able to set up whatever he wants.
For example usage with a local instance of "g4f":

Run g4f container:

docker run -p 8080:8080 -p 1337:1337 -p 7900:7900 --shm-size="2g" hlohaus789/g4f:latest

Update plugin config to use it:

return {
    {
        "piersolenski/wtf.nvim",
        dependencies = {
            "MunifTanjim/nui.nvim",
        },
        config = function ()
            local wtf = require("wtf")
            wtf.setup({
                openai_api_key = "sk-xxxxxxxxxxxxxx",
                openai_model_id = "gpt-3.5-turbo",
                openai_base_url = "http:https://127.0.0.1:1337",
            })
        end
    },
}

[k2589]

@piersolenski Hey! Could you review please?

[piersolenski]

Tiny typo with foun!

[piersolenski]

Can we refactor to not use vim globals?

[piersolenski]

Is the nil check necessary here? As we already have the default API endpoint set in the default_ops as a fallback for if one is not specified? Even if you intentionally set it to be nil in your config, the default will still be set in this case?