Generic client type for OpenAiService

[dsgrieve]

Motivation and Context

Working on Assistants API and the client is an AssistantsAsyncClient, not an OpenAIAsyncClient.

Description

Change OpenAiService so that the client is generic. This also forces a change in OpenAiServiceBuilder to include a generic client type.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄