feat: Callback interface for topic reader

[rekby]

Feature Request

Describe the Feature Request

Available to set callback function as callback for receive new messages. It will allow to easy parallel works

f := func(ctx, batch topicreader.MessageBatch) (action, error){
  return topicreader.ActionCommit, nil
}

reader, err := client.StartReader(ctx, ..., WithCallback(f))

callback f can be called within many goroutines - one goroutine by partition.

it will be mach simpler if need to parallel work.