Support allowNonTransactional config

Add a new producer extended property for allowNonTransactional.
When set to true, records published to this output binding will
not be run in a transaction, unless one is already in process.

By default, all ouput bindings associated with a transactional
binder publishes in a new transaction. This new property can be
used to override this behavior.

Addressing PR review comments.

Resolves https://github.com/spring-cloud/spring-cloud-stream-binder-kafka/issues/990

spring-cloud-stream-binder-kafka-core/src/main/java/org/springframework/cloud/stream/binder/kafka/properties/KafkaProducerProperties.java

@@ -105,6 +105,11 @@ public class KafkaProducerProperties {
 	 */
 	private int closeTimeout;
 
+	/**
+	 * Set to true to disable transactions.
+	 */
+	private boolean allowNonTransactional;
+
 	/**
 	 * @return buffer size
 	 *
@@ -279,6 +284,14 @@ public class KafkaProducerProperties {
 		this.closeTimeout = closeTimeout;
 	}
 
+	public boolean isAllowNonTransactional() {
+		return this.allowNonTransactional;
+	}
+
+	public void setAllowNonTransactional(boolean allowNonTransactional) {
+		this.allowNonTransactional = allowNonTransactional;
+	}
+
 	/**
 	 * Enumeration for compression types.
 	 */