ProxyCommand support (Issue 970)

[knoxg]

This PR adds a new connectVia() method to SocketClient ( the superclass of SSHClient ), which takes an InputStream and OutputStream to use for communications.

When these streams are supplied by a java.lang.Process object, this gives sshj similar functionality to the 'ProxyCommand' directive in openssh, which enables the use of sshj with AWS SSM and EC2 instance connect.

An example class is also provided.