Add support for AssertJ

Add AssertJ as a managed dependency and also include it in
spring-boot-starter-test. Also provide a simple adapter class to allow
Hamcrest matchers to be used as AssertJ Conditions.

Fixes gh-5048

spring-boot-docs/pom.xml

@@ -125,6 +125,11 @@
 			<artifactId>commons-pool2</artifactId>
 			<optional>true</optional>
 		</dependency>
+		<dependency>
+			<groupId>org.assertj</groupId>
+			<artifactId>assertj-core</artifactId>
+			<optional>true</optional>
+		</dependency>
 		<dependency>
 			<groupId>io.dropwizard.metrics</groupId>
 			<artifactId>metrics-core</artifactId>