Python: add parsing of (Azure) OpenAI into classes, including ChatMessages - #2442

Motivation and Context

1. There were a lot of opinions in what is and isn't returned by the different OpenAI classes, that is now much more complete (things like Usage are returned as well)
2. Tricks had to be done to deal with 1 or more results in the openai class, that now moves to downstream classes, or user code.
3. This will also make Function Calling much easier
4. Adds Usage on both service and function level
5. This will make it much easier to store and restore chat messages

One note, most of the completion results and other classes are based on the openai api definitions, which are probably the most prevalent, but a mapping of these and HF and other ai services is needed.

Description

1. Added models folder to store completion results, usage_result, finish_reason, completion_content and appropriate base classes as well as the chat versions of these, and a Role class.
2. Added models into connectors\ai\openai\models to store subclasses of above, with specific classmethods that create these items from openai_objects.
3. Adapted chat_completion_client_base and text_completion_client_base to have ai_service_client_base as the base which houses a logger and usage, adapted all downstream methods for these objects (complete_chat_async and the like)
4. adapted tests for all this and added unit tests for all new classes (except enums)

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 😄