FR: Add `path()` method to `DirEntry`

[netthier]

I noticed that openssh_sftp_client::fs::DirEntry is missing a path() method which is provided in the std-equivalent.
Is there a technical reason this isn't implemented?

[NobodyXu]

It can be implemented, I was focusing on providing a minimal usable API based on what is provided by sftp protocol.

I will welcome any PR for this, you would need to record the path passed to Fs::open_dir and then it can be implemented with relative ease.