.Net Issue Fix - Add FunctionResponse to ChatStreaming interfaces

[RogerBarreto]

Motivation and Context

Resolves #3198

Today we can't call functions using streaming, this change allows it to be used.

The current approach will require the stream to be buffered while listening to a potential function call.

The example shows how that will be achieved.

StringBuilder chatContent = new();

// Non function result streaming will happen here.
await foreach (var message in chatResult.GetStreamingChatMessageAsync())
{
    if (message.Content is not null)
    {
        Console.Write(message.Content);
        chatContent.Append(message.Content);
    }
}
chatHistory.AddAssistantMessage(chatContent.ToString());

// After all stream was loaded check if a `FunctionResponse` was present.
var functionResponse = await chatResult.GetStreamingFunctionResponseAsync();

Remark: Calling GetStreamingFunctionResponseAsync() will buffer the stream to capture the full function call definition, use it after all the streaming messages were flushed out.

Description

Allows streaming interfaces to get the function result.

This is a simplified approach (Buffer the streaming of a function call request until it's complete). Using the FunctionCall

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 😄

[alexminza]

@RogerBarreto it seems that when the chat completion call returns just text and no function call to be invoked - the current code will just throw a NotSupportedException when checking GetOpenAIStreamingFunctionResponseAsync because of https://github.com/microsoft/semantic-kernel/blob/main/dotnet/src/Connectors/Connectors.AI.OpenAI/AzureSdk/SKChatMessage.cs#L40

public FunctionCall FunctionCall
        => this._message?.FunctionCall ?? throw new NotSupportedException("Function call is not supported");