Add direct WebFlux and WebMvc endpoint support

Add `@ControllerEndpoint` and `@RestControllerEndpoint` annotations that
can be used to develop a Spring-only request mapped endpoint. Both
Spring MVC and Spring WebFlux are supported.

This feature is primarily for use when deeper Spring integration is
required or when existing Spring Boot 1.5 projects want to migrate to
Spring Boot 2.0 without re-writing existing endpoints. It comes at the
expense of portability, since such endpoints will be missing from
Jersey.

Fixes gh-10257

spring-boot-project/spring-boot-parent/src/checkstyle/import-control.xml

@@ -43,15 +43,13 @@
 		<disallow pkg="org.springframework.http" />
 		<disallow pkg="org.springframework.web" />
 		<subpackage name="web">
+			<allow pkg="org.springframework.http" />
+			<allow pkg="org.springframework.web" />
 			<subpackage name="servlet">
 				<disallow pkg="org.springframework.web.reactive" />
-				<allow pkg="org.springframework.http" />
-				<allow pkg="org.springframework.web" />
 			</subpackage>
 			<subpackage name="reactive">
 				<disallow pkg="org.springframework.web.servlet" />
-				<allow pkg="org.springframework.http" />
-				<allow pkg="org.springframework.web" />
 			</subpackage>
 		</subpackage>
 	</subpackage>