Support FlushAsync in Producer

[DR9885]

Producer can batch messages with SendAndForgetAsync, but has no way to flush.

Example:

        foreach (var message in messages)
        {
            await producer.SendAndForgetAsync(message);
        }
        await producer.FlushAsync()

[Lanayx]

Java reference

[DR9885]

Any updates on this? It would be super helpful if I could force flush as needed, once I've setup a batch and want to save.

Or if application needed to be shutdown, I'd like to force flush out.

[Lanayx]

This enhancement is approved for PR