Package refactoring in rsocket support

Create annotation.support sub-package and move handler code there. This
prepares for a future, functional handler (responder) variant and is
consistent with the package structure under simp.

spring-messaging/src/test/java/org/springframework/messaging/rsocket/RSocketBufferLeakTests.java

@@ -58,6 +58,7 @@ import org.springframework.core.io.buffer.PooledDataBuffer;
 import org.springframework.messaging.handler.annotation.MessageExceptionHandler;
 import org.springframework.messaging.handler.annotation.MessageMapping;
 import org.springframework.messaging.handler.annotation.Payload;
+import org.springframework.messaging.rsocket.annotation.support.RSocketMessageHandler;
 import org.springframework.stereotype.Controller;
 import org.springframework.util.ObjectUtils;
 

spring-messaging/src/test/java/org/springframework/messaging/rsocket/RSocketClientToServerIntegrationTests.java

@@ -39,6 +39,7 @@ import org.springframework.core.codec.StringDecoder;
 import org.springframework.core.io.buffer.NettyDataBufferFactory;
 import org.springframework.messaging.handler.annotation.MessageExceptionHandler;
 import org.springframework.messaging.handler.annotation.MessageMapping;
+import org.springframework.messaging.rsocket.annotation.support.RSocketMessageHandler;
 import org.springframework.stereotype.Controller;
 
 import static org.assertj.core.api.Assertions.assertThat;

spring-messaging/src/test/java/org/springframework/messaging/rsocket/RSocketServerToClientIntegrationTests.java

@@ -44,6 +44,7 @@ import org.springframework.core.codec.CharSequenceEncoder;
 import org.springframework.core.codec.StringDecoder;
 import org.springframework.core.io.buffer.NettyDataBufferFactory;
 import org.springframework.messaging.handler.annotation.MessageMapping;
+import org.springframework.messaging.rsocket.annotation.support.RSocketMessageHandler;
 import org.springframework.stereotype.Controller;
 
 /**

spring-messaging/src/test/kotlin/org/springframework/messaging/rsocket/RSocketClientToServerCoroutinesIntegrationTests.kt

@@ -37,6 +37,7 @@ import org.springframework.core.codec.StringDecoder
 import org.springframework.core.io.buffer.NettyDataBufferFactory
 import org.springframework.messaging.handler.annotation.MessageExceptionHandler
 import org.springframework.messaging.handler.annotation.MessageMapping
+import org.springframework.messaging.rsocket.annotation.support.RSocketMessageHandler
 import org.springframework.stereotype.Controller
 import reactor.core.publisher.Flux
 import reactor.test.StepVerifier