Add ability to customize "/chat/completions" path after base url

[LukeSamkharadze]

Confirm this is a feature request for the Python library and not the underlying OpenAI API.

• [X] This is a feature request for the Python library

Describe the feature or improvement you're requesting

Some providers have custom endpoint for calling models.

For ex https://api.minimax.chat/v1/text/chatcompletion_v2

If we used python library with base url of https://api.minimax.chat/v1 final request is being made on https://api.minimax.chat/v1/chat/completions which doesn't exist.

Instead of hardcoding path /chat/completions param So lets add option for specifying custom path.

Example:

from openai import OpenAI
client = OpenAI()

completion = client.chat.completions.create(
  model="custom_model",
  messages=[
    {"role": "system", "content": "You are a helpful assistant."},
    {"role": "user", "content": "Hello!"}
  ],
  custom_path="text/chatcompletion_v2"
)

print(completion.choices[0].message)

Additional context

No response