Support for SSH_FXP_REALPATH

[drakkan]

It seems that this library does not allow to query the remote path via sftp. So there are hacks such as this one:

https://github.com/niklas-8/RemoteFiles/blob/master/lib/services/connection_methods.dart#L87

to get the remote path. The problem here is that execute send an SSH exec request. SFTP only servers generally does not accept exec requests but only sftp requests.

In Jsch I think the method to use is realpath

See https://github.com/niklas-8/RemoteFiles/issues/15