.Net: Copy code related to AzureChatCompletionSeervice from Connectors.OpenAI to Connectors.AzureOpenAI

[SergeyMenshykh]

Motivation and Context

As a first step in migrating AzureOpenAIConnector to Azure AI SDK v2, all code related to AzureOpenAIChatCompletionService, including unit tests, is copied from the Connectors.OpenAI project to the Connectors.AzureOpenAI project as-is, with only the structural modifications described below and no logical modifications. This is a preparatory step before refactoring the AzureOpenAIChatCompletionService to use Azure SDK v2.

Description

This PR does the following:

1. Copies the AzureOpenAIChatCompletionService class and all its dependencies to the Connectors.AzureOpenAI project as they are, with no code changes.
2. Copies all existing unit tests related to the AzureOpenAIChatCompletionService service and its dependencies to the Connectors.AzureOpenAI.UnitTests project.
3. Renames some files in the Connectors.AzureOpenAI project so that their names begin with AzureOpenAI instead of OpenAI.
4. Changes namespaces in the copied files from Microsoft.SemanticKernel.Connectors.OpenAI to Microsoft.SemanticKernel.Connectors.AzureOpenAI.

Related to the "Move reusable code from existing Microsoft.SemanticKernel.Connectors.OpenAI project to the new project" task of
the #6864 issue.

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 😄