Replace Azure.AI.OpenAI retry policy with Azure.Core retry policy

[annelo-msft]

This is part of the Core convergence story.

Azure.AI.OpenAI is an SCM-based client, and currently uses an internal retry policy derived from SCM ClientRetryPolicy. The internal type adds support for Azure-specific retry-after headers to the SCM policy. This is a classic example of how Azure.Core types are intended to add Azure-specific functionality over SCM types.

We cannot implement this task until we are able to add Azure.Core policies into SCM pipelines. Once we light up this scenario via the Core convergence work, it would be great to make this change to Azure.AI.OpenAI to tie it into the bigger picture.