GH-3092 Refactor FTP and SFTP outbound specs, handlers

[deepak5127]

Following are the changes,

1. Deprecated SFTP and FTP classes outboundAdapter method which takes RemoteFileTemplate. Instead created new methods in both classes which takes SftpRemoteFileTemplate and FtpRemoteFileTemplate respectively.
2. Deprecated SftpMessageHandlerSpec's constructor which takes RemoteFileTemplate and added new constructors which takes SftpRemoteFileTemplate
3. Deprecated FtpMessageHandlerSpec's constructor which takes RemoteFileTemplate and added new constructors which takes FtpRemoteFileTemplate. Deprecated FtpMessageHandler's constructor which takes RemoteFileTemplate and added new constructor with ftpRemoteFileTemplate.
4. Added new test cases in FtpTests and SftpTests

Fixes #3092

[artembilan]

Please, add your name to the @author list of all the affected classes.

Would you also mind to take a look into the (s)ftp.adoc to be sure that code snippets over there use already an new API.

Great job so far, thank you!

[artembilan]

Please, format JavaDocs properly.
And of course @since 5.4!

[deepak5127]

Added

[deepak5127]

@artembilan Added @author for all the files I have changed. (s)ftp.adoc does not reference any code related to RemoteFileTemplate. Please check all the comments I have incorporated and let me know if I miss something

[artembilan]

There must be a blank line before @since on the class level JavaDocs. This way the author list is much visible.

[artembilan]

@deepak5127 ,

thank you for contribution; looking forward for more!

[deepak5127]

Thanks @artembilan for the guidance. Please also let me know if you have anything you feel I can work on, meanwhile I will check on other open issues.

[artembilan]

Right. See here:

#3194
#3306

Also there are still a lot of tests which should got to JUnit 5 from their JUnit 4.