.Net: OpenAI V2 Connector - ChatCompletion + FC - Phase 06

[RogerBarreto]

Motivation and Context

This PR does the following:

1. Migrates OpenAIChatCompletionService, ClientCore, and other model classes both use, to OpenAI SDK v2.
2. Updates ToolCallBehavior classes to return a list of functions and function choice. This change is required because the new SDK model requires both of those for the CompletionsOptions class creation and does not allow setting them after the class is already created, as it used to allow.
3. Adapts related unit tests to the API changes.
4. Added netstandard2.0 as target for the UnitTest project.
5. Rename TryGetFunctionAndArguments to TryGetOpenAIFunctionAndArguments to avoid clashing with AzureOpenAI