Add support for SCP

[mTsBucy1]

I would like a simple function to download a file from the remote host, e.g. using scp.

I will probably implement this myself, it shouldn't be too difficult.

[samkearney]

I am working on a basic implementation of an SFTP client (upload/download files) in a fork. @Miyoshi-Ryota, just checking, will you accept a PR that adds this feature to the Client?

[Miyoshi-Ryota]

@samkearney Yes! I will accept a PR that adds this feature to the Client. I look forward to your PR very much. Thank you.