fix: Added object mapper bean to fix issue with spring cloud stream 4…

….1.3 failing to deserialize objects (spring-cloud/spring-cloud-stream#2977) causing pubsub steam dead letter test to fail

spring-cloud-gcp-samples/spring-cloud-gcp-pubsub-stream-dead-letter-sample/src/main/java/com/example/SinkExample.java

@@ -16,12 +16,15 @@
 
 package com.example;
 
+import com.fasterxml.jackson.databind.ObjectMapper;
 import com.google.cloud.pubsub.v1.Subscriber;
 import com.google.cloud.spring.pubsub.support.BasicAcknowledgeablePubsubMessage;
 import com.google.cloud.spring.pubsub.support.GcpPubSubHeaders;
 import java.util.function.Consumer;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
+import org.springframework.cloud.function.context.config.JsonMessageConverter;
+import org.springframework.cloud.function.json.JacksonMapper;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.messaging.Message;
@@ -63,4 +66,9 @@ public Consumer<UserMessage> deadLetterMessages() {
             userMessage.getUsername(),
             userMessage.getBody());
   }
+
+  @Bean
+  public JsonMessageConverter customJsonMessageConverter(ObjectMapper objectMapper) {
+    return new JsonMessageConverter(new JacksonMapper(objectMapper));
+  }
 }