JIRA: https://jira.spring.io/browse/INT-4046
Since not all FTP servers provide proper `STAT` command implementation,
plus the `NLIST` doesn't work properly for directories cases, introduce the `FtpRemoteFileTemplate.ExistsMode`
to let:
* to perform `STAT` by default (previous) behavior;
* to switch to `NLIST` for `FtpRemoteFileTemplate` internal use;
* perform the full `NLIST` and `FTPClient.changeWorkingDirectory()` algorithm if needed.
* Improve (S)Ftp components to use proper `RemoteFileTemplate` for internal instantiation
* Introduce `FtpMessageHandler` to wrap `FtpRemoteFileTemplate` with the proper `NLIST` `ExistsMode`
* Cover `NLIST` switching from the `FtpOutboundChannelAdapterParser` and `FtpOutboundGatewayParser`
* Document the `FtpRemoteFileTemplate.ExistsMode`
* Fix typo in the recently introduced `RemoteFileOperations.getSession()` method name
* Add JavaDoc to `Session.exists()`
* Add `NLIST` support for the `FtpSession.exists()` to meet the API requirements
**Cherry-pick to 4.2.x and 4.1.x**
Addressing PR comments
Doc Polishing
c620438913