Support OPENAI_BASE_URL

[codefromthecrypt]

OPENAI_BASE_URL is more canonical and less errorprone (in adrian's opinion) than OPENAI_HOST for indicating the base endpoint. Typically, this ends in /v1. We should take care to also both handle trailing slash and not require it. After this, I think we should sunset OPENAI_HOST so that there's only one way.

from _client.py

        if base_url is None:
            base_url = os.environ.get("OPENAI_BASE_URL")
        if base_url is None:
            base_url = f"https://api.openai.com/v1"

https://github.com/openai/openai-python#configuring-the-http-client