Support for embeddings endpoint

### Description

Reference: https://platform.openai.com/docs/guides/embeddings

Would be helpful for building agents with indexes. Since authentication is already handled, having a one stop shop for all the APIs properly typed in Golang would be nice.

Something like this: https://github.com/charmbracelet/fantasy/commit/cbca0e58119d606e48f4b64dea3c92b217c97a74

Use the OpenAI embeddings guide and the linked fantasy commit as the initial references, then inspect the repository’s existing typed API and provider entry points. Done means the embeddings endpoint is supported consistently with the project’s Go API surface and matches the referenced API behavior.

Written by the indexing model from the issue text.