Provide constructors for injection

Currently when reading/writing JSON and using the reader/marshaller provided
and you want a custom Gson or ObjectMapper instance it still creates the not
needed instance. Move the construction to a constructor and provide a constructor
to directly pass in the pre-configured Gson or ObjectMapper instance.

The same approach is used in Spring itself where Gson or ObjectMapper instances
can be passed in.

spring-batch-infrastructure-tests/src/test/java/org/springframework/batch/item/json/GsonJsonFileItemWriterFunctionalTests.java

@@ -34,9 +34,7 @@ public class GsonJsonFileItemWriterFunctionalTests extends JsonFileItemWriterFun
 	@Override
 	protected JsonObjectMarshaller<Trade> getJsonObjectMarshallerWithPrettyPrint() {
 		Gson gson = new GsonBuilder().setPrettyPrinting().create();
-		GsonJsonObjectMarshaller<Trade> jsonObjectMarshaller = new GsonJsonObjectMarshaller<>();
-		jsonObjectMarshaller.setGson(gson);
-		return jsonObjectMarshaller;
+		return new GsonJsonObjectMarshaller<>(gson);
 	}
 
 	@Override

spring-batch-infrastructure-tests/src/test/java/org/springframework/batch/item/json/JacksonJsonFileItemWriterFunctionalTests.java

@@ -35,9 +35,7 @@ public class JacksonJsonFileItemWriterFunctionalTests extends JsonFileItemWriter
 	protected JsonObjectMarshaller<Trade> getJsonObjectMarshallerWithPrettyPrint() {
 		ObjectMapper objectMapper = new ObjectMapper();
 		objectMapper.enable(SerializationFeature.INDENT_OUTPUT);
-		JacksonJsonObjectMarshaller<Trade> jsonObjectMarshaller = new JacksonJsonObjectMarshaller<>();
-		jsonObjectMarshaller.setObjectMapper(objectMapper);
-		return jsonObjectMarshaller;
+		return new JacksonJsonObjectMarshaller<>(objectMapper);
 	}
 
 	@Override