Collect and display build information

This commit updates the Maven plugin to generate a
`META-INF/boot/build.properties` file with various build-specific
settings (group, artifact, name, version and build time). Additionally,
the plugin can be configured to write an arbitrary number of additional
properties.

A new `BuildProperties` bean is automatically exposed when such a file is
present. If that bean is present, an `InfoContributor` is automatically
created to expose that information under the `build` key.

As for the git contributor, it is possible to only display the core
settings or everything using the `management.info.build.mode` property.

See gh-2559

spring-boot-samples/spring-boot-sample-actuator/src/test/java/sample/actuator/SampleActuatorApplicationTests.java

@@ -147,6 +147,10 @@ public class SampleActuatorApplicationTests {
 		assertThat(entity.getBody())
 				.contains("\"artifact\":\"spring-boot-sample-actuator\"");
 		assertThat(entity.getBody()).contains("\"someKey\":\"someValue\"");
+		assertThat(entity.getBody())
+				.contains("\"java\":{", "\"source\":\"1.8\"", "\"target\":\"1.8\"");
+		assertThat(entity.getBody())
+				.contains("\"encoding\":{", "\"source\":\"UTF-8\"", "\"reporting\":\"UTF-8\"");
 	}
 
 	@Test