Provider options - service_tier for cost saving

[lukaszkn]

Is there a way to pass service_tier parameter to OpenAI API request?

This gives half price compared to standard request:
https://platform.openai.com/docs/guides/flex-processing?api-mode=responses
https://platform.openai.com/docs/pricing?latest-pricing=flex

[michaelw9999]

I just about have this all done done and can make a PR with this feature! I have it working too. Just cleaning up my changes.