GH-7: Add checkstyle and javaformat plugins

Fixes: #7

* Run `./gradlew format`
* Updates from PR review suggestions

function/spring-payload-converter-function/src/test/java/functions/ByteArrayTextToStringTests.java

@@ -32,6 +32,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 public class ByteArrayTextToStringTests {
 
 	private static final String MESSAGE = "hello world";
+
 	private static Function<Message<?>, Message<?>> converter;
 
 	@BeforeAll