Add Support for OpenAI Assistants API to OpenAI plugin

[skryl]

Describe the bug The assistants completion API is slightly different from the base model completions api so the base_url cannot be simply swapped to the endpoint of an Assistant.

To Reproduce

1. Swap OpenAI plugin base_url to an Assistant base url formatted as https://api.openai.com/v1/assistants/{assistant_id}/messages
2. Attempt to use the assistant for completions
3. Receive stack trace around argument mismatch in LLM.chat()

Expected behavior Assistant completions work as expected (similarly to base model completions)

[umaplehurst]

This is already available but it's hiding somewhat.

See docs here: https://github.com/livekit/agents/tree/main/livekit-plugins/livekit-plugins-openai/livekit/plugins/openai/beta/README.md