Refactor TopicClient event loop

[Gazizonoki]

Current example for topic reader:

// Create topic client.

NYdb::NTopic::TTopicClient topicClient(driver);

// Create read session.
NYdb::NTopic::TReadSessionSettings settings;
settings
    .ConsumerName(opts.ConsumerName)
    .AppendTopics(opts.TopicPath);

ReadSession = topicClient.CreateReadSession(settings);

std::cerr << "Session was created" << std::endl;

// [BEGIN read session process events]
// Event loop
while (true) {
    auto future = ReadSession->WaitEvent();
    // Wait for next event or ten seconds
    future.Wait(TDuration::Seconds(10));
    // Get event
    std::optional<NYdb::NTopic::TReadSessionEvent::TEvent> event = ReadSession->GetEvent(true/*block - will block if no event received yet*/);
    std::cerr << "Got new read session event: " << DebugString(*event) << std::endl;

    if (auto* dataEvent = std::get_if<NYdb::NTopic::TReadSessionEvent::TDataReceivedEvent>(&*event)) {
        for (const auto& message : dataEvent->GetMessages()) {
            std::cerr << "Data message: \"" << message.GetData() << "\"" << std::endl;
        }

        if (opts.CommitAfterProcessing) {
            dataEvent->Commit();
        }
    } else if (auto* startPartitionSessionEvent = std::get_if<NYdb::NTopic::TReadSessionEvent::TStartPartitionSessionEvent>(&*event)) {
        startPartitionSessionEvent->Confirm();
    } else if (auto* stopPartitionSessionEvent = std::get_if<NYdb::NTopic::TReadSessionEvent::TStopPartitionSessionEvent>(&*event)) {
        stopPartitionSessionEvent->Confirm();
    } else if (auto* endPartitionSessionEvent = std::get_if<NYdb::NTopic::TReadSessionEvent::TEndPartitionSessionEvent>(&*event)) {
        endPartitionSessionEvent->Confirm();
    } else if (auto* closeSessionEvent = std::get_if<NYdb::NTopic::TSessionClosedEvent>(&*event)) {
        break;
    }
}

It's unfriendly API, we should design more simple and friendly API like ISimpleWriteSession