Make MQTT modules as auto-config

* Fix all the Checkstyle violations for those modules
* Introduce a `MosquittoContainerTest` contract into the `spring-function-test-support`
and use it in the MQTT modules
* Fix READMEs for previously migrated modules to auto-configuration

common/spring-function-test-support/src/main/java/org/springframework/cloud/fn/test/support/mqtt/MosquittoContainerTest.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2021-2024 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.cloud.fn.test.support.mqtt;
+
+import org.junit.jupiter.api.BeforeAll;
+import org.testcontainers.containers.GenericContainer;
+import org.testcontainers.junit.jupiter.Testcontainers;
+
+/**
+ * The base contract for JUnit tests based on the container for MQTT Mosquitto broker. The
+ * Testcontainers 'reuse' option must be disabled,so, Ryuk container is started and will
+ * clean all the containers up from this test suite after JVM exit. Since the Mosquitto
+ * container instance is shared via static property, it is going to be started only once
+ * per JVM, therefore the target Docker container is reused automatically.
+ *
+ * @author Artem Bilan
+ */
+@Testcontainers(disabledWithoutDocker = true)
+public interface MosquittoContainerTest {
+
+	GenericContainer<?> MOSQUITTO_CONTAINER = new GenericContainer<>("eclipse-mosquitto:2.0.13")
+		.withCommand("mosquitto -c /mosquitto-no-auth.conf")
+		.withExposedPorts(1883);
+
+	@BeforeAll
+	static void startContainer() {
+		MOSQUITTO_CONTAINER.start();
+	}
+
+	static String mqttUrl() {
+		return "tcp://localhost:" + MOSQUITTO_CONTAINER.getFirstMappedPort();
+	}
+
+}

common/spring-function-test-support/src/main/java/org/springframework/cloud/fn/test/support/mqtt/package-info.java

@@ -0,0 +1,4 @@
+/**
+ * The MQTT protocol testing support.
+ */
+package org.springframework.cloud.fn.test.support.mqtt;