Add producer transaction API

[ryarnyah]

Inspired from #1527
Fix:

• Transaction Support #1829
• Please add some example of producer transaction! #1760

Changes:

• Add BeginTxn/CommitTxn/AbortTxn/AddOffsetsToTxn/AddMessageToTxn to Sync/Async Producer

Example usage: (for more complete examples see examples/txn_producer && examples/exactly_once)

• Simple transaction producer

config := sarama.NewConfig()
// ...
config.Producer.Transaction.ID = "txn-unique-id"
producer, _ := sarama.NewAsyncProducer(brokers, config)
_ = producer.BeginTxn()
producer.Input() <- &sarama.ProducerMessage{Topic: "test-topic", Key: nil, Value: sarama.StringEncoder("test-value")}
_ = producer.CommitTxn() // Or producer.AbortTxn()

[dnwe]

@ryarnyah thanks — these changes look good to me, especially with both examples and test coverage

I'll look at migrating some of my own applications over to using the transactional produce to exercise the code