Implement service layer

[justinyoo]

Interface:

interface IChatService
{
    IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
        IEnumerable<ChatMessage> messages,
        ChatOptions? options = null,
        CancellationToken cancellationToken = default);
}

Class:

class ChatService(IChatClient chatClient, ILogger<ChatService> logger) : IChatService
{
    public IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
        IEnumerable<ChatMessage> messages,
        ChatOptions? options = null,
        CancellationToken cancellationToken = default)
    {
        // TODO
    }
}