Add support for Spring HATEOAS hypermedia in Actuator endpoints

If spring-hateoas is on the classpath and an MvcEndpoint returns a
@ResponseBody it will be extended and wrapped into a Resource with links.
All the existing endpoints that return sensible JSON data can be extended
this way (i.e. not /logfile). The HAL browser will also be added as an
endpoint if available on the classpath. Finally, asciidocs for the
Actuator endpoints are available as a separate jar file, which if
included in an app will also generate a new (HTTP) endpoint.

Fixes gh-1390

spring-boot-samples/spring-boot-sample-hypermedia/src/main/java/demo/SpringBootHypermediaApplication.java

@@ -0,0 +1,12 @@
+package demo;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class SpringBootHypermediaApplication {
+
+	public static void main(String[] args) {
+		SpringApplication.run(SpringBootHypermediaApplication.class, args);
+	}
+}

spring-boot-samples/spring-boot-sample-hypermedia/src/main/resources/application.properties

@@ -0,0 +1,2 @@
+# management.contextPath=/admin
+endpoints.docs.curies.enabled=true