Reactive Messaging codestart

 * fix #20872
 * created `MyReactiveMessagingApplication` to demonstrates `@Emitter`, `@Incoming` and `@Outgoing` annotations.

devtools/project-core-extension-codestarts/src/main/resources/codestarts/quarkus/extension-codestarts/reactive-messaging-codestart/base/README.tpl.qute.md

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+{#include readme-header /}

devtools/project-core-extension-codestarts/src/main/resources/codestarts/quarkus/extension-codestarts/reactive-messaging-codestart/codestart.yml

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+name: reactive-messaging-codestart
+ref: reactive-messaging-codestart
+tags: extension-codestart
+type: code
+metadata:
+  title: Reactive Messaging codestart
+  description: Use SmallRye Reactive Messaging to interact with Apache Kafka
+  related-guide-section: https://quarkus.io/guides/kafka
+language:
+  base:
+    dependencies:
+      - io.quarkus:quarkus-smallrye-reactive-messaging

devtools/project-core-extension-codestarts/src/main/resources/codestarts/quarkus/extension-codestarts/reactive-messaging-codestart/java/src/main/java/org/acme/MyReactiveMessagingApplication.java

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+package org.acme;
+
+import io.quarkus.runtime.StartupEvent;
+import org.eclipse.microprofile.reactive.messaging.*;
+
+import javax.enterprise.context.ApplicationScoped;
+import javax.enterprise.event.Observes;
+import javax.inject.Inject;
+import java.util.stream.Stream;
+
+@ApplicationScoped
+public class MyReactiveMessagingApplication {
+
+    @Inject
+    @Channel("source")
+    Emitter<String> emitter;
+
+    /** Sends message to the source channel, can be used from a JAX-RS resource or any bean of your application **/
+    void onStart(@Observes StartupEvent ev) {
+        Stream.of("Hello", "with", "SmallRye", "reactive", "message").forEach(string -> emitter.send(string));
+    }
+
+    /** Consume the message from the source channel, uppercase it and send it to the uppercase channel **/
+    @Incoming("source")
+    @Outgoing("uppercase")
+    public Message<String> toUpperCase(Message<String> message) {
+        return message.withPayload(message.getPayload().toUpperCase());
+    }
+
+    /** Consume the uppercase channel and print the message **/
+    @Incoming("uppercase")
+    public void sink(String word) {
+        System.out.println(">> " + word);
+    }
+}