Make config and config defaults accessible to clients

In the new Kafka producer API, the ProducerConfig is not accessible to the clients. Samza uses the ProducerConfig instance to access the defaults property values, which can then be used in the various helper utils. Config instance is accessible even without instantiating a Kafka producer.
With the new API, there is no way to instantiate a ProducerConfig as the constructor is marked private. Also, it does not make the default config values accessible to the client without actually instantiating a KafkaProducer.

Changes suggested:
1. Make the ProducerConfig constructor public
2. Make ConfigDef in ProducerConfig accessible by the client
3. Use public static variables for kafka config default "values"