Add seed parameter for `chat/completions` endpoint

[haukelicht]

Thank you for your work on this package, @irudnyts! My students really love it because it enables them to make their first steps in using LLMs for text analysis task.

If I may, I'd like to make a feature request/contribution:

• OpenAI introduced the seed parameter quite a while ago (https://platform.openai.com/docs/api-reference/chat/create#chat-create-seed).
• I would like to have this added/add this to create_chat_completions() as an argument (with default 42).

I have cloned your repo and created a local branch, "add_seed_parameter", that implements this change (in the source code and test). Would you like to give me permission to push this change?