Allow writing to SshCommand

[blenderfreaky]

By default you can't write to the Standard Input of a running SshCommand and can only write to a process running in SSH by using CreateShellSession. This causes a few problems in my use-case (sending protobuf-formatted packets back and forth):

• The input is mirrored back in the output, useful in an interactive context but not for automation
• Sending arbitrary binary data can cause problems (like accidentally sending \u001b which is escape in UTF16 or similar)
• A newline needs to be sent in order to actually commit input to the program

I've created a workaround for this that uses evil reflection hacks for sending stuff directly to SshCommand.

Proposed changes

• Expose IChannelSession, IChannelSession and SshCommand._channel as public

or

• Add a public method to SshCommand that calls _channel.SendData(byte[] data, int offset, int size)

I could create a PR for either solution, although I'm not sure yet how I would write a test for this.

[Rob-Hague]

This was done in #1293 by way of SshCommand.CreateInputStream (https://sshnet.github.io/SSH.NET/examples.html#stream-data-to-a-command)