GH-1387: Make PartitioningInterceptor as public

Fixes spring-cloud/spring-cloud-stream#1387

To be able to distinguish the `PartitioningInterceptor` in the
`MessageChannel` and remove it from the interceptor it would be better
to have its class as `public`

In some use-case like AWS Kinesis Binder it is important do not have
an SCSt partitioning logic, since the whole functionality is done
on the target system.

**Cherry-pick to 2.0.x**

spring-cloud-stream/src/main/java/org/springframework/cloud/stream/binding/MessageConverterConfigurer.java

@@ -357,7 +357,7 @@ public class MessageConverterConfigurer implements MessageChannelAndSourceConfig
 	/**
 	 *
 	 */
-	protected final class PartitioningInterceptor extends ChannelInterceptorAdapter {
+	public final class PartitioningInterceptor extends ChannelInterceptorAdapter {
 
 		private final BindingProperties bindingProperties;