Method to set the openai_root_

[aschet]

Describe the feature or improvement you're requesting

Currently the API endpoint is hardcoded to "https://api.openai.com/v1". This makes it hard to use this library against other implementations of the OpenAI API, e.g. Ollama, which provides support for chat completion. Therefore, it would be great to add a way to configure a specific endpoint so that this library can be used with Ollama without modification. To my knowledge, this is currently only possible by modifying openai_root_ at compiletime. It also would be great if it were possible to specify one API key per "connection" as it is in the Python API.

Additional context

No response