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

INT-3412 Polishing

- Addressed PR comments
- Completed SFTP implementation
- Added namespace/parser support for `FileExistsMode` (append, etc)
- Added SFTP Tests
- Created Embedded SFTP server for tests (similar to FTP)
- Converted tests that needed a real server to use the embedded server

INT-3412 Docs and Polish (PR Comments)