Add support for FileJob->truncate() in smb/sftp slaves

Summary: This implements the new method added in D26148 in the sftp/smb slaves.

Test Plan:
Tested sftp locally with truncate. Seems to work.
Haven't tested locally with smb though.

Reviewers: sitter, dfaure, fvogt

Reviewed By: sitter, dfaure, fvogt

Subscribers: kde-frameworks-devel, kfm-devel, ngraham

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D26191