GH-2806: Pulsar binder defaults properties config

- Ensure that the Pulsar binder default properties can be properly
   expressed via spring.cloud.stream.pulsar.default property prefix.
 - Add the binder child context bean with the name binderName_binderProducingContext
   into the parent application context so that individual beans from the binder context
   can be easily queried.

Resolves https://github.com/spring-cloud/spring-cloud-stream/issues/2806

core/spring-cloud-stream/src/main/java/org/springframework/cloud/stream/binder/DefaultBinderFactory.java

@@ -482,6 +482,10 @@ public class DefaultBinderFactory implements BinderFactory, DisposableBean, Appl
 
 		if (refresh) {
 			binderProducingContext.refresh();
+			if (this.context != null) {
+				this.context.getBeanFactory().registerSingleton(configurationName + "_binderProducingContext",
+					binderProducingContext);
+			}
 			if (!useApplicationContextAsParent || "integration".equals(binderType.getDefaultName())) {
 				this.propagateSharedBeans((GenericApplicationContext) this.context, binderProducingContext);
 			}