Expose raw http response in model API response

[dumbbellcode]

Summary

• This feature makes the raw http response accessible in text response, so that it can be used to access provider specific information not usually accessible but the current generic API
• Supports Feature: Expose the underlying model API response headers #636

Usage

$response = $agent->prompt('Summarise this document.')
$httpResponse = $response->raw // accessible from the last api call