implement ssh proxy using libssh for jump hosts

The OpenSSH supports ProxyJump command, which invokes another ssh binary to log in to the jump box and do the IO forwarding to the final host. LibSSH implements this still using OpenSSH's ssh binary, which is not great for a library (but it is quite transparent to what users are expecting).

It would be great to support this use case standalone by running separate session inside of libssh to avoid dependency on openssh (either in thread of in separate process).