.Net: Split ClientCore class

[SergeyMenshykh]

Motivation and Context

The ClientCore class has absorbed functionality relevant to different Azure OpenAI-related services. As a first step to reduce its size, it makes sense to split it into small chunks, where each chunk would be related to a service that uses it. Later, we can move those chunks to the services themselves.

Description

This PR does the following:

1. Does not change any logic in any of the services.
2. Splits the ClientCore class into ClientCore.ChatCompletion and ClientCore.Embeddings files.
3. Refactors the AzureOpenAIChatCompletionService and AzureOpenAITextEmbeddingGenerationService to use the relevant ClientCore pieces.
4. Removes the AzureOpenAIClientCore class