Expose a way to customize the path to the chat completions endpoint

[Kerollmops]

Some APIs are close to being compatible with this crate. The byot (bring your own type) feature is handy, but useless when the path is incompatible.

For example, Ollama's chat completions route is close to the OpenAI API but reachable on the /api/chat route. On the other hand, Claude (Anthropic) is available through the /v1/messages route, which is also very close to being compatible with the OpenAI API.