[OpenAI] [Scenario] Add abstractions for chat conversations

[joseharriaga]

From @trrwilson:

ChatClient client = GetTestClient();

ChatConversation conversation = new(client)
{
    InitialMessages =
    [
        ChatRequestMessage.CreateSystemMessage(
            "You are a culinary expert who speaks like a stereotypical caricature of a New Yorker."),
    ],
    Functions = new ChatFunctions(typeof(CulinaryDatabaseTools)),
};

ChatCompletion chatCompletion = conversation.CompleteChat("What are some of the best pizza toppings?");
chatCompletion = conversation.CompleteChat("Which of those are vegetarian-friendly?");
chatCompletion = conversation.CompleteChat("Are any of those spicy?");

conversation.Reset();
chatCompletion = conversation.CompleteChat("What's the best way to cook a pumpkin pie?");

[pallavit]

What benefit does ChatConversation give us over ChatCompletion? Is the usecase purely enabling support for post processing - so function calls?