Feature Description

OpenAI /chat/completion API returns some "usage" information.
While this library makes some of that available in Usage.kt, the prompt_tokens_details and completion_tokens_details fields are missing.
Also see OpenAI doc for field details.

It would be great to make those details available.

Problem it Solves

Particular the prompt_tokens_details contains information about cached_tokens, which informs if OpenAI's caching is working as expected to avoid unexpected costs.

Proposed Solution

Add the missing fields to Usage.kt and read them from OpenAI's response.