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/pom.xml

@@ -71,6 +71,22 @@
 				<configuration>
 					<executable>true</executable>
 				</configuration>
+				<executions>
+					<execution>
+						<id>generate build info</id>
+						<goals>
+							<goal>generate-build-info</goal>
+						</goals>
+						<configuration>
+							<additionalProperties>
+								<encoding.source>${project.build.sourceEncoding}</encoding.source>
+								<encoding.reporting>${project.reporting.outputEncoding}</encoding.reporting>
+								<java.source>${maven.compiler.source}</java.source>
+								<java.target>${maven.compiler.target}</java.target>
+							</additionalProperties>
+						</configuration>
+					</execution>
+				</executions>
 			</plugin>
 		</plugins>
 	</build>