Add getProperty to ProducerSettings

[mdedetrich]

This PR adds the equivalent getProperty from ConsumerSettings (see

alpakka-kafka/core/src/main/scala/akka/kafka/ConsumerSettings.scala

Lines 333 to 336 in eb5246d

	/**
	* Java API: Get a raw property. `null` if it is not defined.
	*/
	def getProperty(key: String): String = properties.getOrElse(key, null)

) into ProducerSettings

[ennru]

LGTM.

[ennru]

Thank you!