INT-3412 (S)FTP Append, rmdir, Client Access

JIRA: https://jira.spring.io/browse/INT-3412

Initial commit - review only.

TODO:
- SFTP Tests
- Namespace/Adapter support for file append
- Docs
:+1: 
Actually, all changes look good. I think you can go ahead with other improvements.
When I see something, I'll let you know