Simplify RSocket client responder config

Now that responder RSocketStrategies also exposes responder strategies,
AnnotationClientResponderConfigurer is reduced and no longer needs to
be public. This commit folds it into RSocketMessageHandler as a nested
class and exposes it as a ClientRSocketFactoryConfigurer through a
static method that accepts the handlers to use.

Effectively a shortcut for creating RSocketMessageHandler, giving it
RSocketStrategies, calling afterPropertiesSet, and then the instance
createResponder.

See gh-23314

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

@@ -39,7 +39,6 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.messaging.handler.annotation.MessageMapping;
 import org.springframework.messaging.rsocket.annotation.ConnectMapping;
-import org.springframework.messaging.rsocket.annotation.support.AnnotationClientResponderConfigurer;
 import org.springframework.messaging.rsocket.annotation.support.RSocketMessageHandler;
 import org.springframework.stereotype.Controller;
 
@@ -111,7 +110,7 @@ public class RSocketServerToClientIntegrationTests {
 						factory.metadataMimeType("text/plain");
 						factory.setupPayload(ByteBufPayload.create("", connectionRoute));
 					})
-					.rsocketFactory(AnnotationClientResponderConfigurer.withHandlers(new ClientHandler()))
+					.rsocketFactory(RSocketMessageHandler.clientResponder(new ClientHandler()))
 					.rsocketStrategies(context.getBean(RSocketStrategies.class))
 					.connectTcp("localhost", server.address().getPort())
 					.block();