Add Prediction field

[danielpeng2]

Describe the change

Adding the prediction field which allows for using Predicted Outputs.

Provide OpenAI documentation link
https://platform.openai.com/docs/api-reference/chat/create#chat-create-prediction

Describe your solution
Adds the optional parameter and modifies the response to include how many prediction tokens it used

Tests
Briefly describe how you have tested these changes. If possible — please add integration tests.
Tested locally and verified that I received predicted tokens accepted in the response

Issue: #954

[Copilot]

Pull Request Overview

This PR adds support for predicted outputs by introducing a new prediction field in the chat completion request and updating usage metrics to track predicted token counts.

• Adds AcceptedPredictionTokens and RejectedPredictionTokens to the Usage metrics in common.go.
• Introduces a Prediction configuration and corresponding struct in chat.go for predicted outputs.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
common.go	Adds new fields to track accepted and rejected predicted tokens.
chat.go	Introduces Prediction field in ChatCompletionRequest and defines Prediction struct.