GH-2788: Add MongoDbChangeStreamMessageProducer

Fixes https://github.com/spring-projects/spring-integration/issues/2788

* Introduce a `MessageProducerSupport.subscribeToPublisher(Publisher<Message<?>>)`
for components which produces `Flux` for data from their source
* Such a component is auto-stopped when subscription to that `Publisher` is canceled
* Implement a `MongoDbChangeStreamMessageProducer` based on the reactive support for
in Spring Data MongoDb
* Implement a Java DSL for `MongoDbChangeStreamMessageProducer`
* Disable a test for change stream since it requires server of version 4.x started with 'replSet' option
* Add `MongoHeaders` for change stream events

* Change `MessageProducerSupport` to use a `takeWhile((message) -> isRunning())`
instead of storing a `subscription` from a callback
* Document new features

* Remove trailing whitespaces

* Doc Polishing.

spring-integration-core/src/test/java/org/springframework/integration/endpoint/ReactiveMessageProducerTests.java

@@ -0,0 +1,92 @@
+/*
+ * Copyright 2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.integration.endpoint;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.integration.channel.FluxMessageChannel;
+import org.springframework.integration.config.EnableIntegration;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.support.GenericMessage;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.junit.jupiter.SpringJUnitConfig;
+
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+
+/**
+ * @author Artem Bilan
+ *
+ * @since 5.3
+ */
+@SpringJUnitConfig
+@DirtiesContext
+public class ReactiveMessageProducerTests {
+
+	@Autowired
+	public FluxMessageChannel fluxMessageChannel;
+
+	@Autowired
+	public MessageProducerSupport producer;
+
+	@Test
+	public void test() {
+		assertThat(this.producer.isRunning()).isTrue();
+
+		StepVerifier.create(
+				Flux.from(this.fluxMessageChannel)
+						.map(Message::getPayload)
+						.cast(String.class))
+				.expectNext("test1", "test2")
+				.thenCancel()
+				.verify();
+
+		assertThat(this.producer.isRunning()).isFalse();
+	}
+
+	@Configuration
+	@EnableIntegration
+	public static class Config {
+
+		@Bean
+		public FluxMessageChannel fluxMessageChannel() {
+			return new FluxMessageChannel();
+		}
+
+		@Bean
+		public MessageProducerSupport producer() {
+			MessageProducerSupport producer =
+					new MessageProducerSupport() {
+
+						@Override
+						protected void doStart() {
+							subscribeToPublisher(Flux.just("test1", "test2").map(GenericMessage::new));
+						}
+
+					};
+			producer.setOutputChannel(fluxMessageChannel());
+			return producer;
+		}
+
+	}
+
+}