Be able to override :base_url

[preciz]

These days one codebase usually uses more than one OpenAI compatible API provider at the same time.

This PR provides the ability to override the base URL.

Example usage:

msgs = [%{role: :user, content: "Hello!"}]

ExOpenAI.Chat.create_chat_completion(msgs, "hf:mistralai/Mistral-7B-Instruct-v0.3",
  base_url: "https://cheapai.local",
  openai_api_key: "1234"
)

[preciz]

Solves #24

[dvcrn]

Thanks for the PR! Looking good!

I'll try to get this released within the next few days ❤️

[dvcrn]

Released in 1.7 - https://github.com/dvcrn/ex_openai/releases/tag/1.7.0