.Net: Migrate Azure Chat Completion Service to AzureOpenAI SDK v2 - #6984

Motivation and Context

This PR is the next step in a series of follow-up PRs to migrate AzureOpenAIConnector to Azure AI SDK v2. It updates all code related to AzureOpenAI ChatCompletionService to use the Azure AI SDK v2. One of the goals of the PR is to update the code with a minimal number of changes to make the code review as easy as possible, so almost all methods keep their names as they were even though they might not be relevant anymore. This will be fixed in one of the follow-up PRs.

Description

This PR does the following:

1. Migrates AzureOpenAIChatCompletionService, ClientCore, and other model classes both use, to Azure AI 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.

Next steps

1. Add integration tests.
2. Rename internal/private methods that were intentionally left with old, irrelevant names to minimize the code review delta.

Out of scope:

• .Net: Support image(url) content by AzureOpenAIChatCompletion service #6991

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 😄