Add OpenAI-compatible embedding client to vmcp optimizer

[gabrielcosi]

The vmcp optimizer only speaks TEI for embeddings. Anything OpenAI-shaped is unreachable.

[jerm-dro]

Thanks for raising this, @gabrielcosi. Adding an OpenAI-compatible embedding backend alongside the existing TEI backend would be fairly self-contained. We would welcome a contribution here -- if you want to take a shot at it, feel free to open a PR and tag us for early feedback.

[gabrielcosi]

@jerm-dro opened #5633 for this one. Let me know if it's on the right track

[jerm-dro]

Thanks @gabrielcosi! I'll take a look today.