Support for SCP protocol

[XVilka]

There exists russh-sftp crate for providing SFTP support on top of russh, but many services still do not support SFTP have have it disabled, while still supporting legacy SCP protocol. Int the Rust ecosystem, the only library that supports it seems to be ssh2: https://docs.rs/ssh2/latest/ssh2/#upload-a-file

Would be nice to support the same feature in russh as well.