Add websocket-authentication sample

websocket-authentication/src/main/java/com/example/greeting/GreetingController.java

@@ -0,0 +1,26 @@
+package com.example.greeting;
+
+import java.time.Duration;
+
+import reactor.core.publisher.Flux;
+
+import org.springframework.graphql.data.method.annotation.QueryMapping;
+import org.springframework.graphql.data.method.annotation.SubscriptionMapping;
+import org.springframework.security.core.Authentication;
+import org.springframework.stereotype.Controller;
+
+@Controller
+public class GreetingController {
+
+	@QueryMapping
+	String greeting(Authentication authentication) {
+		return "Hello " + authentication.getName() + "!";
+	}
+
+	@SubscriptionMapping
+	Flux<String> greetings(Authentication authentication) {
+		return Flux.interval(Duration.ofMillis(50))
+			.map((num) -> "Hello " + authentication.getName() + num + "!");
+	}
+
+}