Add Apache MINA SftpEventListener

- republish certain events as `ApplicationEvent`s.

* * Add ApacheMinaFtplet to provide the same functionality with FTP
* Fix typo

* * Polishing javadocs and event toString() methods

spring-integration-file/src/main/java/org/springframework/integration/file/remote/server/FileServerEvent.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2019 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.file.remote.server;
+
+import org.springframework.integration.events.IntegrationEvent;
+
+/**
+ * Base class for file server events. Typically, the source for these events will be some
+ * kind of client/server session object containing information such as the client's ip
+ * address.
+ *
+ * @author Gary Russell
+ * @since 5.2
+ *
+ */
+@SuppressWarnings("serial")
+public abstract class FileServerEvent extends IntegrationEvent {
+
+	public FileServerEvent(Object source) {
+		super(source);
+	}
+
+	public FileServerEvent(Object source, Throwable cause) {
+		super(source, cause);
+	}
+
+}

spring-integration-file/src/main/java/org/springframework/integration/file/remote/server/package-info.java

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+/**
+ * Provides classes related to file servers.
+ */
+package org.springframework.integration.file.remote.server;