Add custom LayoutFactory sample

Add a sample that shows how a custom LayoutFactory can be used with
both Maven and Gradle.

Closes gh-7263

spring-boot-samples/pom.xml

@@ -32,6 +32,7 @@
 		<module>spring-boot-sample-atmosphere</module>
 		<module>spring-boot-sample-batch</module>
 		<module>spring-boot-sample-cache</module>
+		<module>spring-boot-sample-custom-layout</module>
 		<module>spring-boot-sample-data-cassandra</module>
 		<module>spring-boot-sample-data-couchbase</module>
 		<module>spring-boot-sample-data-elasticsearch</module>