Refine Coroutines annotated controller support

This commit refines Coroutines annotated controller support
by considering Kotlin Unit as Java void and using the right
ReactiveAdapter to support all use cases, including suspending
functions that return Flow (usual when using APIs like WebClient).

It also fixes RSocket fire and forget handling and adds related tests
for that use case.

Closes gh-24057
Closes gh-23866

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

@@ -39,6 +39,7 @@ 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.core.publisher.ReplayProcessor
 import reactor.test.StepVerifier
 import java.time.Duration
 
@@ -50,6 +51,34 @@ import java.time.Duration
  */
 class RSocketClientToServerCoroutinesIntegrationTests {
 
+	@Test
+	fun fireAndForget() {
+		Flux.range(1, 3)
+				.concatMap { requester.route("receive").data("Hello $it").send() }
+				.blockLast()
+		StepVerifier.create(context.getBean(ServerController::class.java).fireForgetPayloads)
+				.expectNext("Hello 1")
+				.expectNext("Hello 2")
+				.expectNext("Hello 3")
+				.thenAwait(Duration.ofMillis(50))
+				.thenCancel()
+				.verify(Duration.ofSeconds(5))
+	}
+
+	@Test
+	fun fireAndForgetAsync() {
+		Flux.range(1, 3)
+				.concatMap { i: Int -> requester.route("receive-async").data("Hello $i").send() }
+				.blockLast()
+		StepVerifier.create(context.getBean(ServerController::class.java).fireForgetPayloads)
+				.expectNext("Hello 1")
+				.expectNext("Hello 2")
+				.expectNext("Hello 3")
+				.thenAwait(Duration.ofMillis(50))
+				.thenCancel()
+				.verify(Duration.ofSeconds(5))
+	}
+
 	@Test
 	fun echoAsync() {
 		val result = Flux.range(1, 3).concatMap { i -> requester.route("echo-async").data("Hello " + i!!).retrieveMono(String::class.java) }
@@ -70,6 +99,16 @@ class RSocketClientToServerCoroutinesIntegrationTests {
 				.verify(Duration.ofSeconds(5))
 	}
 
+	@Test
+	fun echoStreamAsync() {
+		val result = requester.route("echo-stream-async").data("Hello").retrieveFlux(String::class.java)
+
+		StepVerifier.create(result)
+				.expectNext("Hello 0").expectNextCount(6).expectNext("Hello 7")
+				.thenCancel()
+				.verify(Duration.ofSeconds(5))
+	}
+
 	@Test
 	fun echoChannel() {
 		val result = requester.route("echo-channel")
@@ -106,6 +145,19 @@ class RSocketClientToServerCoroutinesIntegrationTests {
 	@Controller
 	class ServerController {
 
+		val fireForgetPayloads = ReplayProcessor.create<String>()
+
+		@MessageMapping("receive")
+		fun receive(payload: String) {
+			fireForgetPayloads.onNext(payload)
+		}
+
+		@MessageMapping("receive-async")
+		suspend fun receiveAsync(payload: String) {
+			delay(10)
+			fireForgetPayloads.onNext(payload)
+		}
+
 		@MessageMapping("echo-async")
 		suspend fun echoAsync(payload: String): String {
 			delay(10)
@@ -123,6 +175,18 @@ class RSocketClientToServerCoroutinesIntegrationTests {
 			}
 		}
 
+		@MessageMapping("echo-stream-async")
+		suspend fun echoStreamAsync(payload: String): Flow<String> {
+			delay(10)
+			var i = 0
+			return flow {
+				while(true) {
+					delay(10)
+					emit("$payload ${i++}")
+				}
+			}
+		}
+
 		@MessageMapping("echo-channel")
 		fun echoChannel(payloads: Flow<String>) = payloads.map {
 			delay(10)
@@ -185,8 +249,6 @@ class RSocketClientToServerCoroutinesIntegrationTests {
 
 		private lateinit var server: CloseableChannel
 
-		private val interceptor = FireAndForgetCountingInterceptor()
-
 		private lateinit var requester: RSocketRequester
 
 
@@ -196,7 +258,6 @@ class RSocketClientToServerCoroutinesIntegrationTests {
 			context = AnnotationConfigApplicationContext(ServerConfig::class.java)
 
 			server = RSocketFactory.receive()
-					.addResponderPlugin(interceptor)
 					.frameDecoder(PayloadDecoder.ZERO_COPY)
 					.acceptor(context.getBean(RSocketMessageHandler::class.java).responder())
 					.transport(TcpServerTransport.create("localhost", 7000))